Fix build errors with WITH_HEADLESS or WITH_GHOST_SDL

Disable WITH_XR_OPENXR entirely for these cases. For headless XR
features don't make much sense, for SDL support is not implemented.

CMakeLists.txt

@@ -684,6 +684,7 @@ if(WITH_GHOST_SDL OR WITH_HEADLESS)
   set(WITH_X11_ALPHA     OFF)
   set(WITH_GHOST_XDND    OFF)
   set(WITH_INPUT_IME     OFF)
+  set(WITH_XR_OPENXR     OFF)
 endif()
 
 if(WITH_CPU_SSE)