wayland-pointer: Bail unconditionally if without native backend

Using -Dnative_backend=false caused build failure due to a missing (implicit) definition of META_IS_BACKEND_X11. But if we define it properly then that just leaves some of the function's locals uninitialized and it will never work anyway. Just return unconditionally if there's no native backend to initialize the variables.

Closes: #1025 (closed)