net/rfkill/rfkill.c: fix unused rfkill_led_trigger() warning

commit 4dec9b80

 ("rfkill: strip pointless
notifier chain") removed the only user of rfkill_led_trigger() that was not
guarded by #ifdef CONFIG_RFKILL_LEDS. Therefore, move rfkill_led_trigger()
completely inside #ifdef CONFIG_RFKILL_LEDS and avoid the compile time
warning:

net/rfkill/rfkill.c:59: warning: 'rfkill_led_trigger' defined but not used

Signed-off-by: Simon Holm Thøgersen <odie@cs.aau.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>