xwm: Initialize X selection on X server startup

This way an already existing wayland selection will be available on
launch.
diff --git a/src/xwayland/selection.c b/src/xwayland/selection.c
index 83ea2b3..f10032e 100644
--- a/src/xwayland/selection.c
+++ b/src/xwayland/selection.c
@@ -683,4 +683,6 @@
 	seat = &wm->server->compositor->seat->seat;
 	wm->selection_listener.notify = weston_wm_set_selection;
 	wl_signal_add(&seat->selection_signal, &wm->selection_listener);
+
+	weston_wm_set_selection(&wm->selection_listener, seat);
 }