.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 <jdelvare@suse.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Joe Perches <joe@perches.com>
Cc: Jonathan Corbet <corbet@lwn.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>