They are also needed by the MMSH and MMST protocols and therefore
the file they are in is pulled in when these protocols are enabled
and used. By moving them to a separate file, linking statically to
libavformat while only using AVIO no longer pulls in all the
muxers/demuxers (and also no longer any AVCodecs when linking
statically to libavcodec).
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>