mac80211: check for CONFIG_MAC80211_LEDS in the tpt_led_trigger declaration

If CONFIG_MAC80211_LEDS is not set, ieee80211_i.h was failing to compile,
because struct led_trigger is only declared when CONFIG_LEDS_TRIGGERS is
set.

This patch adds ifdefs around the tpt_led_trigger declaration in
ieee80211_i.h to avoid the problem.

Signed-off-by: Luciano Coelho <luciano.coelho@nokia.com>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>