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

The commit 1a33e10e ("net: partially revert dynamic lockdep key
changes") reverts the commit ab92d68f ("net: core: add generic lockdep
keys"). But it forgot to also revert the commit 5759af06 ("batman-adv:
Drop lockdep.h include for soft-interface.c") which depends on the latter.

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