batman-adv: Drop usage of export.h

The linux/export.h include was introduced in commit 9bcb94c8
("batman-adv: Introduce missing headers for genetlink restructure") to have
access to THIS_MODULE. But with commit 5b20755b ("init: move
THIS_MODULE from <linux/export.h> to <linux/init.h>"), it was moved and the
include for export.h is no longer needed.

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