batman-adv: add 'crc16' dependency which is still needed but got removed accidentally

The 'crc16' kernel lib is still needed/used by the BLA framework
(Bridge Loop Avoidance) in bridge_loop_avoidance.c. Thanks to
Antonio 'ordex' Quartulli for confirmation.

Introduced by 1ee5b17e4f7b2c145524edfb51617c911fad7cc4
("upgrade package to latest release 2014.0.0").

Signed-off-by: Bastian Bittorf <bittorf@bluebottle.com>
Signed-off-by: Marek Lindner <mareklindner@neomailbox.ch>