Prefix all _demuxer, _muxer, _protocol from libavformat and libavdevice.

This also lists the objects from those two libraries as internal (by adding
the ff_ prefix) so that they can then be hidden via linker scripts.
(cherry picked from commit c6610a216e)

libavformat/oma.c

@@ -189,7 +189,7 @@ static int oma_read_probe(AVProbeData *p)
 }
 
 
-AVInputFormat oma_demuxer = {
+AVInputFormat ff_oma_demuxer = {
     "oma",
     NULL_IF_CONFIG_SMALL("Sony OpenMG audio"),
     0,