.gitattributes: set git diff driver for C source code files

Git can be told to apply language-specific rules when generating diffs.
Enable this for C source code files (*.c and *.h) so that function names
are printed right.  Specifically, doing so prevents "git diff" from
mistakenly considering unindented goto labels as function names.

Link: http://lkml.kernel.org/r/20160907143403.1449324f@endymion
Signed-off-by: Jean Delvare <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Joe Perches <[email protected]>
Cc: Jonathan Corbet <[email protected]>
Signed-off-by: Andrew Morton <[email protected]>
Signed-off-by: Linus Torvalds <[email protected]>

.gitattributes

@@ -0,0 +1,2 @@
+*.c   diff=cpp
+*.h   diff=cpp