xmptag: Call gst_tag_register_musicbrainz_tags during init
We need to call this to register the MusixBrainz tags before we use them in an XMP schema.
Fixes this critical when attempting to run jpegparse on a JPEG containing MusicBrainz XMP tags:
GStreamer-CRITICAL **: 20:41:07.885: gst_tag_get_type: assertion 'info != NULL' failed