Backport of "xmptag: Call gst_tag_register_musicbrainz_tags during init" into 1.20
Backport of !3092 (merged) "xmptag: Call gst_tag_register_musicbrainz_tags during init".
Commits:
- 4edfc86c xmptag: Call gst_tag_register_musicbrainz_tags during init