mtd: map.h: fix arm cross-build failure

This patch fixes the following build failure:
In file included from include/linux/mtd/qinfo.h:4:0,
                 from include/linux/mtd/pfow.h:7,
                 from drivers/mtd/lpddr/lpddr_cmds.c:27:
include/linux/mtd/map.h: In function 'inline_map_read':
include/linux/mtd/map.h:409:3: error: implicit declaration of function 'BUILD_BUG_ON' [-Werror=implicit-function-declaration]
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>

include/linux/mtd/map.h

@@ -26,7 +26,7 @@
 #include <linux/list.h>
 #include <linux/string.h>
 #include <linux/bug.h>
-
+#include <linux/kernel.h>
 
 #include <asm/unaligned.h>
 #include <asm/system.h>