x86 platform drivers: make Dell laptop driver select needed LED support

Otherwise we get compile errors like this:

  ERROR: "led_classdev_unregister" [drivers/platform/x86/dell-laptop.ko] undefined!
  ERROR: "led_classdev_register" [drivers/platform/x86/dell-laptop.ko] undefined!
  make[1]: *** [__modpost] Error 1
  make: *** [modules] Error 2

when the dell-laptop support is enabled without the necessary LED
support being enabled.

Reported-by: Alessandro Suardi <alessandro.suardi@gmail.com>
Acked-by: Matthew Garrett <mjg59@srcf.ucam.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>