batman-adv: init neigh node last seen field

Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>
[sven@narfation.org: fix conflicts with current version]
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Antonio Quartulli <a@unstable.cc>