batman-adv: Add missing include for in_interrupt()

The fix for receiving (internally generated) bla packets outside the
interrupt context introduced the usage of in_interrupt(). But this
functionality is only defined in linux/preempt.h which was not included
with the same patch.

Fixes: 279e89b2 ("batman-adv: bla: use netif_rx_ni when not in interrupt context")
Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>

net/batman-adv/bridge_loop_avoidance.c

@@ -25,6 +25,7 @@
 #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/seq_file.h>