drm/xe: Add missing include xe_wait_user_fence.h

Make xe_wait_user_fence.c include xe_wait_user_fence.h so it doesn't
rely on indirect includes and also doesn't fail the build due to missing
prototype for xe_wait_user_fence_ioctl().

Signed-off-by: Lucas De Marchi <lucas.demarchi@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>