Fixes this build error when RFKILL is not set:

drivers/platform/x86/sony-laptop.c:1050: undefined reference to `rfkill_unregister'
and so on..
Signed-off-by: NAlexander Beregalov <a.beregalov@gmail.com>
Acked-by: NMatthew Garrett <mjg@redhat.com>
Signed-off-by: NMattia Dongili <malattia@linux.it>
Signed-off-by: NLen Brown <len.brown@intel.com>