rpi3: Fix compilation error when stack protector is enabled

Include necessary header file to use ARRAY_SIZE() macro

Change-Id: I5b7caccd02c14c598b7944cf4f347606c1e7a8e7
Signed-off-by: Madhukar Pappireddy <madhukar.pappireddy@arm.com>

plat/rpi3/rpi3_stack_protector.c

 /*
- * Copyright (c) 2017-2018, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2017-2019, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
@@ -7,6 +7,7 @@
 #include <stdint.h>
 
 #include <lib/utils.h>
+#include <lib/utils_def.h>
 
 #include "rpi3_private.h"