libavformat: Build hevc.o when building the RTP muxer

The RTP muxer enables the actual codepaths within sdp.c,
which depend on hevc.o since e5cfc8fd.

This fixes builds with --disable-everything --enable-muxer=rtp.

Signed-off-by: Martin Storsjö <martin@martin.st>
Merge commit 'fccfc22d1f304aef42a0b960e4c1d55ce67107f5'

* commit 'fccfc22d1f304aef42a0b960e4c1d55ce67107f5':
  libavformat: Build hevc.o when building the RTP muxer

Merged-by: Michael Niedermayer <michaelni@gmx.at>
Allow mov musing if the ac3 parser was disabled.

This reverts 8091fe30.
Reported and tested by Takashi, ffmpeg-list dubistmeinheld de