message: Work around g-i/pygobject/gjs bug with ~0 in enums
GST_MESSAGE_ANY was considered a long by pygobject and gjs, and thus couldn't be used in gst_bus_poll() and similar APIs as they expect an int-typed enum. Just use 0xffffffff instead for now. https://bugzilla.gnome.org/show_bug.cgi?id=732633