gst-play: call gst_deinit()

So we can use gst-play to track memory leaks.

https://bugzilla.gnome.org/show_bug.cgi?id=765216

tools/gst-play.c

@@ -1289,5 +1289,6 @@ main (int argc, char **argv)
   g_free (video_sink);
 
   g_print ("\n");
+  gst_deinit ();
   return 0;
 }