set flac_metadata_respond(VORBIS_COMMENT) so that flac responds to vorbis comments and returns these to the metadata callback (so we can properly set the replaygain state)

git-svn-id: https://svn.musicpd.org/mpd/trunk@1707 09075e82-0dd4-0310-85a5-a0d7c8717e4f