mac80211: declare ieee80211_ave_rssi as EXPORT

ieee80211_ave_rssi need to be declare as export for driver to use it.

Signed-off-by: Wey-Yi Guy <wey-yi.w.guy@intel.com>
Signed-off-by: John W. Linville <linville@tuxdriver.com>
diff --git a/net/mac80211/util.c b/net/mac80211/util.c
index 577942b..8ba8b49 100644
--- a/net/mac80211/util.c
+++ b/net/mac80211/util.c
@@ -1803,3 +1803,4 @@
 
 	return ifmgd->ave_beacon_signal;
 }
+EXPORT_SYMBOL_GPL(ieee80211_ave_rssi);