compal-laptop: Make it depend on CONFIG_RFKILL

-tip testing found this build failure (x86 randconfig):

 drivers/built-in.o: In function `setup_rfkill':
 compal-laptop.c:(.text+0x36abe8): undefined reference to `rfkill_alloc'
 compal-laptop.c:(.text+0x36abfc): undefined reference to `rfkill_register'
 compal-laptop.c:(.text+0x36ac30): undefined reference to `rfkill_alloc'
 compal-laptop.c:(.text+0x36ac44): undefined reference to `rfkill_register'

Which can happen with CONFIG_COMPAL_LAPTOP=y but COMPAL_LAPTOP=m.
Signed-off-by: Ingo Molnar <mingo@elte.hu>

drivers/platform/x86/Kconfig

@@ -179,6 +179,7 @@ config COMPAL_LAPTOP
 	tristate "Compal Laptop Extras"
 	depends on ACPI
 	depends on BACKLIGHT_CLASS_DEVICE
+	depends on RFKILL
 	---help---
 	  This is a driver for laptops built by Compal: