Include what we use.

Change-Id: If510431d3d445de94f781b4fdec2e1b445f7c294
diff --git a/base/strings.cpp b/base/strings.cpp
index 5f7eccc..6f698d9 100644
--- a/base/strings.cpp
+++ b/base/strings.cpp
@@ -17,6 +17,7 @@
 #include "base/strings.h"
 
 #include <stdlib.h>
+#include <string.h>
 
 #include <string>
 #include <vector>