Fix MISRA rule 8.4 in common code

Rule 8.4: A compatible declaration shall be visible when
          an object or function with external linkage is defined.

Change-Id: I26e042cb251a6f9590afa1340fdac73e42f23979
Signed-off-by: Roberto Vargas <roberto.vargas@arm.com>
diff --git a/lib/stdlib/abort.c b/lib/stdlib/abort.c
index af19ccf..65ce4cc 100644
--- a/lib/stdlib/abort.c
+++ b/lib/stdlib/abort.c
@@ -1,10 +1,11 @@
 /*
- * Copyright (c) 2013-2014, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2013-2018, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
 
 #include <debug.h>
+#include <stdlib.h>
 
 /*
  * This is a basic implementation. This could be improved.