avformat/vorbiscomment: Replace AVDictionary ** by const AVDictionary *

avformat/vorbiscomment: Replace AVDictionary ** by const AVDictionary *

ff_vorbiscomment_write() used an AVDictionary ** parameter for a
dictionary whose contents ought to be written; yet this can be replaced
by AVDictionary * since commit 042ca05f0fdc5f4d56a3e9b94bc9cd67bca9a4bc;
and this in turn can be replaced by const AVDictionary * to indicate
that the dictionary isn't modified; the latter also applies to
ff_vorbiscomment_length().

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>