gitattributes: Cover Objective-C source files

Apple's Git distribution actually carries a similar file which
annotates *.m:
https://github.com/apple-opensource/Git/blob/73/gitattributes

See comments in commit 29cf16d ("buildsys: Help git-diff
adding .gitattributes config file") for details.

Signed-off-by: Philippe Mathieu-Daudé <[email protected]>
Reviewed-by: Christian Schoenebeck <[email protected]>
Message-Id: <[email protected]>

Author: philmd

.gitattributes

@@ -1,3 +1,4 @@
 *.c.inc         diff=c
 *.h.inc         diff=c
+*.m             diff=objc
 *.py            diff=python