Add support of TAG_MUSICBRAINZ_RELEASEGROUPID

Closes #1767

src/db/plugins/ProxyDatabasePlugin.cxx

@@ -182,6 +182,8 @@ static constexpr struct {
 #endif
 #if LIBMPDCLIENT_CHECK_VERSION(2,21,0)
 	{ TAG_MOOD, MPD_TAG_MOOD },
+	{ TAG_MUSICBRAINZ_RELEASEGROUPID,
+	  MPD_TAG_MUSICBRAINZ_RELEASEGROUPID },
 #endif
 	{ TAG_NUM_OF_ITEM_TYPES, MPD_TAG_COUNT }
 };