batman-adv: Switch to kstrtox.h for kstrtou64

The commit 4c527293 ("kernel.h: split out kstrtox() and simple_strtox()
to a separate header") moved the kstrtou64 function to a new header called
linux/kstrtox.h.

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