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>