[Perl/perl5] bc0177: Override *.h files as C with Linguist

[Elvin Aslanov via perl5-changes]

  Branch: refs/heads/blead
  Home:   https://github.com/Perl/perl5
  Commit: bc01770b32f9d9bf9d313d2eec370c41822cd61c
      
https://github.com/Perl/perl5/commit/bc01770b32f9d9bf9d313d2eec370c41822cd61c
  Author: Elvin Aslanov <rwp.prim...@gmail.com>
  Date:   2023-01-15 (Sun, 15 Jan 2023)

  Changed paths:
    M .gitattributes

  Log Message:
  -----------
  Override *.h files as C with Linguist

GitHub classifies 23 files as C++ for some reason.
https://github.com/Perl/perl5/search?q=language%3AC%2B%2B&type=code
I believe Perl doesn't contain C++ code, and C++ headers can have a 
distinguishable .hh, .hpp, .hxx, and .h++ extensions.