cfg80211: export cfg80211_wext_siwfreq

cfg80211_wext_siwfreq() should be exported with EXPORT_SYMBOL_GPL.

Signed-off-by: Jussi Kivilinna <jussi.kivilinna@mbnet.fi>
Acked-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>