batman-adv: Allow selecting BATMAN V if CFG80211 is not built

With the new stub for cfg80211_get_station(), we can now build the
BATMAN V protocol even with a kernel that was built without any
wireless support.

Signed-off-by: Linus Lüssing <linus.luessing@c0d3.blue>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>