Override *.inc language classification (#267)

`alpha-premul-table.inc` is classified as PHP, that amounts to 13.4% of the code:

https://user-images.githubusercontent.com/33230602/38056427-12c3094c-32e5-11e8-89db-0684b5608859.png

With this override, linguist will classify the inc file as C.

.gitattributes

@@ -1,2 +1,2 @@
 *.sh	crlf=input
-
+*.inc	linguist-language=C