s390/kasan: make kasan header self-contained

It is relying on _REGION1_SHIFT / _REGION2_SHIFT values which come from
asm/pgtable.h, so include it.
Reviewed-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>
Signed-off-by: Heiko Carstens <hca@linux.ibm.com>

arch/s390/include/asm/kasan.h

@@ -2,6 +2,8 @@
 #ifndef __ASM_KASAN_H
 #define __ASM_KASAN_H
 
+#include <asm/pgtable.h>
+
 #ifdef CONFIG_KASAN
 
 #define KASAN_SHADOW_SCALE_SHIFT 3