Fix buildkernel (missing include).

Reported-by: luxh
kernel/ath: Grab the WLAN serializer around ieee80211_ifdetach().

Reported-by: Imre Vadasz <imre@vdsz.com>
kernel/802_11: Release the WLAN serializer around if_detach().

This should fix all crashes we are seeing upon kldunload of WLAN
drivers.

In-discussion-with-and-tested-by: ivadasz, jh32, sephe