Fix unexpected LinkageSpec error

[PaulWagener]

I encountered errors in a static release build of OpenCV on Windows. These changes fixed those errors.

[twistedfall]

Thanks for the PR! Before we merge that, can you please check if the version 0.88.2 of the crate doesn't fix the error you're getting? I have fixed the problem in walker.rs that was triggered by clang-17 a couple of days ago and my impression is that change in element.rs is not required at least for the issue that was reported initially (#519).

If version 0.88.2 works for you then I'd say let's keep it at this, otherwise I will merge this PR.

[PaulWagener]

The problem indeed occurs in 0.88.2.

I triple check already because I saw that same issue and thought I was losing my mind on how I keep encountering an error that was supposedly already fixed.

Below is a backtrace of the kind of error that this PR fixes.
Note that this error orginates from walk_opencv_entities while the original fix was for visit_cv_namespace

 thread 'main' panicked at D:\Source\opencv-rust\binding-generator\src\walker.rs:57:33:
  internal error: entered unreachable code: Unsupported decl for file: Entity {
      kind: LinkageSpec,
      display_name: None,
      location: Some(
          SourceLocation {
              file: Some(
                  File {
                      path: "D:\\Source\\.cargo\\registry\\src\\index.crates.io-6f17d22bba15001f\\opencv-0.88.2\\src_cpp\\ocvrs_common.hpp",
                  },
              ),
              line: 37,
              column: 8,
              offset: 1306,
          },
      ),
  }
  stack backtrace:
     0:     0x7ff639390063 - std::sys_common::backtrace::_print::impl$0::fmt
                                 at /rustc/de686cbc65478db53e3d51c52497685e852cc092/library\std\src\sys_common\backtrace.rs:44
     1:     0x7ff6393b200d - core::fmt::rt::Argument::fmt
                                 at /rustc/de686cbc65478db53e3d51c52497685e852cc092/library\core\src\fmt\rt.rs:142
     2:     0x7ff6393b200d - core::fmt::write
                                 at /rustc/de686cbc65478db53e3d51c52497685e852cc092/library\core\src\fmt\mod.rs:1120
     3:     0x7ff63938b3b1 - std::io::Write::write_fmt<std::sys::windows::stdio::Stderr>
                                 at /rustc/de686cbc65478db53e3d51c52497685e852cc092/library\std\src\io\mod.rs:1810
     4:     0x7ff63938fe6a - std::sys_common::backtrace::_print
                                 at /rustc/de686cbc65478db53e3d51c52497685e852cc092/library\std\src\sys_common\backtrace.rs:47
     5:     0x7ff63938fe6a - std::sys_common::backtrace::print
                                 at /rustc/de686cbc65478db53e3d51c52497685e852cc092/library\std\src\sys_common\backtrace.rs:34
     6:     0x7ff6393925c9 - std::panicking::default_hook::closure$1
                                 at /rustc/de686cbc65478db53e3d51c52497685e852cc092/library\std\src\panicking.rs:272
     7:     0x7ff63939228b - std::panicking::default_hook
                                 at /rustc/de686cbc65478db53e3d51c52497685e852cc092/library\std\src\panicking.rs:292
     8:     0x7ff639392ab4 - std::panicking::rust_panic_with_hook
                                 at /rustc/de686cbc65478db53e3d51c52497685e852cc092/library\std\src\panicking.rs:779
     9:     0x7ff639392985 - std::panicking::begin_panic_handler::closure$0
                                 at /rustc/de686cbc65478db53e3d51c52497685e852cc092/library\std\src\panicking.rs:657
    10:     0x7ff639390959 - std::sys_common::backtrace::__rust_end_short_backtrace<std::panicking::begin_panic_handler::closure_env$0,never$>
                                 at /rustc/de686cbc65478db53e3d51c52497685e852cc092/library\std\src\sys_common\backtrace.rs:171
    11:     0x7ff639392694 - std::panicking::begin_panic_handler
                                 at /rustc/de686cbc65478db53e3d51c52497685e852cc092/library\std\src\panicking.rs:645
    12:     0x7ff6393ba0c7 - core::panicking::panic_fmt
                                 at /rustc/de686cbc65478db53e3d51c52497685e852cc092/library\core\src\panicking.rs:72
    13:     0x7ff638df62a4 - <I as core::iter::traits::collect::IntoIterator>::into_iter::hca1942f9d8e7cd23
    14:     0x7ff638df58e2 - <I as core::iter::traits::collect::IntoIterator>::into_iter::hca1942f9d8e7cd23
    15:     0x7ff63929ea2c - clang::Entity::visit_children::visit::h3ca34c698472c061

[twistedfall]

Thanks a lot for checking!

[twistedfall]

This has been released in 0.88.4 crate version