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/exit.c b/lib/stdlib/exit.c
index 3d23d7b..afc3f93 100644
--- a/lib/stdlib/exit.c
+++ b/lib/stdlib/exit.c
@@ -1,10 +1,11 @@
 /*
- * Copyright (c) 2015, ARM Limited and Contributors. All rights reserved.
+ * Copyright (c) 2015-2018, ARM Limited and Contributors. All rights reserved.
  *
  * SPDX-License-Identifier: BSD-3-Clause
  */
 
 #include <debug.h>
+#include <stdlib.h>
 
 void exit(int v)
 {