include: kernel.h: include printk.h

Adding "printk.h" will help improve portability from linux kernel
code (in my case, lib/asn1_decoder.c and others) where printf and
pr_* variant functions are used.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>

include/linux/kernel.h

 #ifndef _LINUX_KERNEL_H
 #define _LINUX_KERNEL_H
 
-
 #include <linux/types.h>
+#include <linux/printk.h> /* for printf/pr_* utilities */
 
 #define USHRT_MAX	((u16)(~0U))
 #define SHRT_MAX	((s16)(USHRT_MAX>>1))