Commit 0d1229f1 factored the main part
of the voc demuxer's read_packet function out; yet when this Libav
commit was merged in f99195d5, the
dependency of the other users of this function on vocdec.o was
unnecessarily kept. This commit fixes this.

While just at it, also disable the data only used by the voc demuxer
and muxer in voc.c if both of them are disabled.
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>