gst/typefind/gsttypefindfunctions.c: Terminate vararg functions with NULL instead of 0 to make gcc4 happy.

Original commit message from CVS:
* gst/typefind/gsttypefindfunctions.c: (mp3_type_find),
(mpeg2_sys_type_find), (mpeg1_sys_type_find),
(mpeg_video_type_find), (mpeg_video_stream_type_find):
Terminate vararg functions with NULL instead of 0 to
make gcc4 happy.