gst-libs/gst/xoverlay/xoverlay.*: replace XID with unsigned long to get rid of the xlibs dependency in

Original commit message from CVS:
* gst-libs/gst/xoverlay/xoverlay.c: (gst_x_overlay_set_xwindow_id),
(gst_x_overlay_got_xwindow_id):
* gst-libs/gst/xoverlay/xoverlay.h:
replace XID with unsigned long to get rid of the xlibs dependency in
XOverlay (fixes #137004)