batman-adv: Add new include for min/max helpers

[ Upstream commit fcd193e1 ]

The commit b296a6d5 ("kernel.h: split out min()/max() et al. helpers")
moved the min/max helper functionality from kernel.h to minmax.h. Adjust
the kernel code accordingly to avoid fragile indirect includes.

Signed-off-by: Sven Eckelmann <sven@narfation.org>
Signed-off-by: Simon Wunderlich <sw@simonwunderlich.de>
Stable-dep-of: 8c8ecc98 ("batman-adv: Drop unmanaged ELP metric worker")
Signed-off-by: Sasha Levin <sashal@kernel.org>