These fallback definitions (since they were moved to avcommon_compat.h
in 5e192e1e) caused the avcodec plugin to use av_metadata_get for uses
of av_dict_get, while av_metadata_get is defined in libavformat (which
the avcodec plugin doesn't link to).

Both AVDictionaryEntry and av_dict_get were added in libavutil
51.5.0, which is earlier than the minimum required version of libavutil
that vlc requires (51.22.0).

This fixes using the avcodec plugin in builds with libavformat < 54
(such as on debian wheezy).

This is not cherry-picked from vlc.git, since the master branch there
has dropped support for such old libavformat versions altogether.
Signed-off-by: Martin Storsjö <martin@martin.st>