qtdemux: support timed-text subtitle tracks.

https://bugzilla.gnome.org/show_bug.cgi?id=752818
diff --git a/gst/isomp4/qtdemux_types.c b/gst/isomp4/qtdemux_types.c
index 8e8189d..dc71228 100644
--- a/gst/isomp4/qtdemux_types.c
+++ b/gst/isomp4/qtdemux_types.c
@@ -201,6 +201,7 @@
   {FOURCC_schi, "scheme information", QT_FLAG_CONTAINER},
   {FOURCC_pssh, "protection system specific header", 0},
   {FOURCC_tenc, "track encryption", 0},
+  {FOURCC_stpp, "XML subtitle sample entry", 0},
   {0, "unknown", 0,},
 };