xwm: Set _NET_WM_CM_S0 instead of relying on the xwayland module to do it

Not sure why I made xwayland claim this selection, seem a little
awkward in retrospect.
diff --git a/src/xwayland/xwayland.h b/src/xwayland/xwayland.h
index c68a517..21b499e 100644
--- a/src/xwayland/xwayland.h
+++ b/src/xwayland/xwayland.h
@@ -85,6 +85,7 @@
 		xcb_atom_t		 wm_state;
 		xcb_atom_t		 wm_s0;
 		xcb_atom_t		 wm_client_machine;
+		xcb_atom_t		 net_wm_cm_s0;
 		xcb_atom_t		 net_wm_name;
 		xcb_atom_t		 net_wm_pid;
 		xcb_atom_t		 net_wm_icon;