.gitattributes: *.inc recognized as Pawn

Prevent that from happening in GH language stats.

It seems only `Math-BigInt` tests use `*.inc` for Perl code

https://github.com/search?q=repo%3APerl%2Fperl5%20path%3A*.inc&type=code

Author: rwp0

.gitattributes

@@ -26,3 +26,5 @@ warnings.h linguist-generated
 *.t linguist-language=Perl
 *.h linguist-language=C
 *.fnc linguist-language=Text
+*.inc linguist-language=C
+cpan/Math-BigInt/t/*.inc linguist-language=Perl