cfg80211: Add stub for cfg80211_get_station()

This allows modules using this function (currently: batman-adv) to
compile even if cfg80211 is not built at all, thus relaxing
dependencies.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Johannes Berg <johannes.berg@intel.com>

include/net/cfg80211.h

@@ -1102,6 +1102,7 @@ struct station_info {
 	struct cfg80211_tid_stats pertid[IEEE80211_NUM_TIDS + 1];
 };
 
+#if IS_ENABLED(CONFIG_CFG80211)
 /**
  * cfg80211_get_station - retrieve information about a given station
  * @dev: the device where the station is supposed to be connected to
@@ -1114,6 +1115,14 @@ struct station_info {
  */
 int cfg80211_get_station(struct net_device *dev, const u8 *mac_addr,
 			 struct station_info *sinfo);
+#else
+static inline int cfg80211_get_station(struct net_device *dev,
+				       const u8 *mac_addr,
+				       struct station_info *sinfo)
+{
+	return -ENOENT;
+}
+#endif
 
 /**
  * enum monitor_flags - monitor flags