xwm.h was meant to be private, so move it to include/xwayland/xwm.h
We had an ifdef WLR_HAS_XCB_ICCCM in xwayland.h which was easy to move
to xwm, it is not safe to use the WLR_HAS_* in the public headers.
I checked a few of our current users and none rely on xwm.h being
public as expected (rootston, sway, hsroots)
