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 29cf16db23 ("buildsys: Help git-diff
adding .gitattributes config file") for details.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Christian Schoenebeck <qemu_oss@crudebyte.com>
Message-Id: <20220317130326.39188-1-philippe.mathieu.daude@gmail.com>

.gitattributes

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