memtest: remove unused header files

memtest does not require these headers to be included.
Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Cc: Leon Romanovsky <leon@leon.nu>
Acked-by: David Rientjes <rientjes@google.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/memtest.c

 #include <linux/kernel.h>
-#include <linux/errno.h>
-#include <linux/string.h>
 #include <linux/types.h>
-#include <linux/mm.h>
-#include <linux/smp.h>
 #include <linux/init.h>
-#include <linux/pfn.h>
 #include <linux/memblock.h>
 
 static u64 patterns[] __initdata = {