Add support for the TPOS/TPA (disc number) id3 tag.
diff --git a/include/media/mediametadataretriever.h b/include/media/mediametadataretriever.h
index 113c452..022b849 100644
--- a/include/media/mediametadataretriever.h
+++ b/include/media/mediametadataretriever.h
@@ -54,6 +54,7 @@
     METADATA_KEY_VIDEO_WIDTH     = 20,
     METADATA_KEY_WRITER          = 21,
     METADATA_KEY_MIMETYPE        = 22,
+    METADATA_KEY_DISC_NUMBER     = 23,
     // Add more here...
 };