COMPMID-2486: Remove/add disabled compiler warnings

Removed the following flags:
-Wno-vla
-Wno-strict-overflow

Added:
-Wformat-security

Change-Id: I49eb3d724e14db796e543164295674617c37cb65
Signed-off-by: Michalis Spyrou <michalis.spyrou@arm.com>
Reviewed-on: https://review.mlplatform.org/c/2109
Tested-by: Arm Jenkins <bsgcomp@arm.com>
Comments-Addressed: Arm Jenkins <bsgcomp@arm.com>
Reviewed-by: Michele Di Giorgio <michele.digiorgio@arm.com>
diff --git a/tests/validation/reference/Erode.cpp b/tests/validation/reference/Erode.cpp
index 5e8b36f..e7598c3 100644
--- a/tests/validation/reference/Erode.cpp
+++ b/tests/validation/reference/Erode.cpp
@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2017 ARM Limited.
+ * Copyright (c) 2017-2019 ARM Limited.
  *
  * SPDX-License-Identifier: MIT
  *
@@ -51,7 +51,8 @@
     */
     SimpleTensor<T> dst(src.shape(), src.data_type());
 
-    for(int i = 0; i < src.num_elements(); ++i)
+    const uint32_t num_elements = src.num_elements();
+    for(uint32_t i = 0; i < num_elements; ++i)
     {
         Coordinates coord = index2coord(src.shape(), i);
         const int   x     = coord.x();