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: Alexander Beregalov <a.beregalov@gmail.com>
Acked-by: Matthew Garrett <mjg@redhat.com>
Signed-off-by: Mattia Dongili <malattia@linux.it>
Signed-off-by: Len Brown <len.brown@intel.com>