batman-adv: Drop unused header preempt.h

The commit b1de0f01 ("batman-adv: Use netif_rx_any_context().") removed
the last user for a function declaration from linux/preempt.h. The include
should therefore be cleaned up.
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>

net/batman-adv/bridge_loop_avoidance.c

@@ -25,7 +25,6 @@
 #include <linux/lockdep.h>
 #include <linux/netdevice.h>
 #include <linux/netlink.h>
-#include <linux/preempt.h>
 #include <linux/rculist.h>
 #include <linux/rcupdate.h>
 #include <linux/skbuff.h>