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.

Author: rwp0

.gitattributes

@@ -21,3 +21,4 @@ warnings.h linguist-generated
 *.pl linguist-language=Perl
 *.pm linguist-language=Perl
 *.t linguist-language=Perl
+*.h linguist-language=C