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>
diff --git a/net/wireless/wext-compat.c b/net/wireless/wext-compat.c
index 1ff1e9f..bb5e0a5 100644
--- a/net/wireless/wext-compat.c
+++ b/net/wireless/wext-compat.c
@@ -1471,6 +1471,7 @@
 		return -EOPNOTSUPP;
 	}
 }
+EXPORT_SYMBOL_GPL(cfg80211_wext_siwpmksa);
 
 static const iw_handler cfg80211_handlers[] = {
 	[IW_IOCTL_IDX(SIOCGIWNAME)]	= (iw_handler) cfg80211_wext_giwname,