Pacify unused function warning in xfns.c with GTK2

Pacify unused function warning in xfns.c with GTK2

* src/xfns.c (x_get_net_workarea, x_get_monitor_for_frame)
(x_make_monitor_attribute_list, x_get_monitor_attributes_fallback):
[HAVE_XINERAMA] (x_get_monitor_attributes_xinerama)
[HAVE_XRANDR] (x_get_monitor_attributes_xrandr)
(x_get_monitor_attributes): Fix #ifdefs around definitions to avoid
unused function warnings regardless of GTK use (bug#46509).
[HAVE_XRANDR] (x_get_monitor_attributes_xrandr): Undefine
RANDR13_LIBRARY after it's been used.