batman-adv: Drop lockdep.h include for soft-interface.c

The commit ab92d68f ("net: core: add generic lockdep keys") removed
all lockdep functionality from soft-interface.c but didn't remove the
include for this functionality.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>