clock: define AUTO_CLEANUP_FREE_FUNC for GstClockID
GstClockID is secretly a gpointer so we can't use g_autoptr(), instead user can do: g_auto (GstClockID) clock_id = 0;
GstClockID is secretly a gpointer so we can't use g_autoptr(), instead user can do: g_auto (GstClockID) clock_id = 0;