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>

net/batman-adv/gateway_common.c

@@ -10,7 +10,7 @@
 #include <linux/atomic.h>
 #include <linux/byteorder/generic.h>
 #include <linux/errno.h>
-#include <linux/kernel.h>
+#include <linux/kstrtox.h>
 #include <linux/limits.h>
 #include <linux/math64.h>
 #include <linux/netdevice.h>