Fixes stdlib.h dependencies in test suites

Moved stdlib.h in test suites, so platforms that don't support
MBEDTLS_PLATFORM_C would build.
diff --git a/tests/suites/helpers.function b/tests/suites/helpers.function
index edf1d12..d12be75 100644
--- a/tests/suites/helpers.function
+++ b/tests/suites/helpers.function
@@ -2,11 +2,12 @@
 /*----------------------------------------------------------------------------*/
 /* Headers */
 
+#include <stdlib.h>
+
 #if defined(MBEDTLS_PLATFORM_C)
 #include "mbedtls/platform.h"
 #else
 #include <stdio.h>
-#include <stdlib.h>
 #define mbedtls_printf     printf
 #define mbedtls_fprintf    fprintf
 #define mbedtls_calloc    calloc