batman-adv: enable B.A.T.M.A.N. V compilation by default

Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
Acked-by: Antonio Quartulli <a@unstable.cc>
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>