Add support for the OriginalDate tag

See https://picard.musicbrainz.org/docs/mappings/

This tag is useful when the user would like all releases of the same
album to be sorted next to each other.

src/db/plugins/ProxyDatabasePlugin.cxx

@@ -158,6 +158,9 @@ static constexpr struct {
 	{ TAG_NAME, MPD_TAG_NAME },
 	{ TAG_GENRE, MPD_TAG_GENRE },
 	{ TAG_DATE, MPD_TAG_DATE },
+#if LIBMPDCLIENT_CHECK_VERSION(2,12,0)
+	{ TAG_ORIGINAL_DATE, MPD_TAG_ORIGINAL_DATE },
+#endif
 	{ TAG_COMPOSER, MPD_TAG_COMPOSER },
 	{ TAG_PERFORMER, MPD_TAG_PERFORMER },
 	{ TAG_COMMENT, MPD_TAG_COMMENT },