Fix language statistics by considering *.inc files to be C files. (#1078)

Otherwise, they are considered to be PHP or C++ files, which causes GitHub's
language statistics to show that Capstone is written mostly in PHP (42.2%) and
C++ (33.1%), which is nonsense.
diff --git a/.gitattributes b/.gitattributes
new file mode 100644
index 0000000..03e638d
--- /dev/null
+++ b/.gitattributes
@@ -0,0 +1 @@
+/arch/**/*.inc linguist-language=C