Avoid warning on unknown upper_case_acronyms lint on old clippys

[dtolnay]

The clippy::upper_case_acronyms lint is newly introduced in the Clippy that shipped with rustc 1.51.0. When building CXX-generated Rust code with a toolchain older than 1.51.0, this PR avoids a clippy::unknown_clippy_lints warning in the generated code.

warning: unknown clippy lint: clippy::upper_case_acronyms
 --> path/to/bridge.rs:7:1
  |
7 | #[cxx::bridge]
  | ^^^^^^^^^^^^^^
  |
  = note: `#[warn(clippy::unknown_clippy_lints)]` on by default
  = help: for further information visit https://rust-lang.github.io/rust-clippy/master/index.html#unknown_clippy_lints
  = note: this warning originates in an attribute macro (in Nightly builds, run with -Z macro-backtrace for more info)