Move static function fill_filter_caches() from h264.h to h264.c.

The function is only used within that file, so it makes sense to place
it ther. This fixes many warnings of the type:
h264.h:1170: warning: ‘fill_filter_caches’ defined but not used


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@22876 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b