mac80211: remove stray extern

Somehow this snuck into my earlier patch, and
only now did I see a compiler warning:

net/mac80211/led.c:218:13: warning: function '__ieee80211_create_tpt_led_trigger' with external linkage has definition

Remove the stray extern.

Signed-off-by: Johannes Berg <johannes.berg@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>