wireless: correct sparse warning in wext-compat.c

  CHECK   net/wireless/wext-compat.c
net/wireless/wext-compat.c:1434:5: warning: symbol 'cfg80211_wext_siwpmksa' was not declared. Should it be static?

Add declaration in cfg80211.h.  Also add an EXPORT_SYMBOL_GPL, since all
the peer functions have it.

Signed-off-by: John W. Linville <linville@tuxdriver.com>