Allow selecting only the vc1 demuxer.

This fixes the following option set:
  --disable-encoders --disable-decoders --disable-parsers --disable-bsfs
  --disable-muxers --disable-demuxers --disable-protocols
  --enable-demuxer=vc1

Originally committed as revision 9649 to svn://svn.ffmpeg.org/ffmpeg/trunk