Only enable "-pedantic-errors" when BENCHMARK_ENABLE_WERROR is ON

[BraynStorm]

Newer versions of clang (on Windows at least) fail to compile benchmark with -Werror and -pedantic-errors.
Unfortunately, the BENCHMARK_ENABLE_WERROR option toggles only -Werror, and not -pedantic-errors.

.../build/Debug/_deps/benchmark-src/src/benchmark.cc:225:7: error: offset of on non-standard-layout type 'State' [-Werror,-Winvalid-offsetof]
   225 |       offsetof(State, skipped_) <= (cache_line_size - sizeof(skipped_)), "");
       |       ^               ~~~~~~~~
 C:\Program Files\LLVM\lib\clang\18\include\__stddef_offsetof.h:11:24: note: expanded from macro 'offsetof'
    11 | #define offsetof(t, d) __builtin_offsetof(t, d)
       |                        ^                     ~
 1 error generated.
 ninja: build stopped: subcommand failed.

This PR changes the CMakeLists.txt file to only append -pedantic-errors when BENCHMARK_ENABLE_WERROR is ON

[google-cla]

Thanks for your pull request! It looks like this may be your first contribution to a Google open source project. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

View this failed invocation of the CLA check for more information.

For the most up to date status, view the checks section at the bottom of the pull request.

[LebedevRI]

You'll need to deal with the CLA check...

[dmah42]

closing as unresponded.