Include what you use

mbstowcs needs stdlib.h header on mipsel

BUG=

Change-Id: Ia93d0a877b2e22d5c1c461f0dea9999ede725520
Reviewed-on: https://chromium-review.googlesource.com/354020
Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>
diff --git a/src/common/string_utils.cpp b/src/common/string_utils.cpp
index 73da759..02bccd0 100644
--- a/src/common/string_utils.cpp
+++ b/src/common/string_utils.cpp
@@ -9,6 +9,8 @@
 
 #include "string_utils.h"
 
+#include <stdlib.h>
+
 #include <fstream>
 #include <sstream>