batman-adv: Add missing include for in_interrupt()

[ Upstream commit 4bba9dab86b6ac15ca560ef1f2b5aa4529cbf784 ]

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: 279e89b2281a ("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>
Signed-off-by: Sasha Levin <sashal@kernel.org>