Fix build break after a0efaec

Change-Id: Ie8c77a53a3daeef61527ba4544cae417ade16fef
diff --git a/NatController.h b/NatController.h
index cf059e1..2912e4b 100644
--- a/NatController.h
+++ b/NatController.h
@@ -19,6 +19,7 @@
 
 #include <linux/in.h>
 #include <list>
+#include <string>
 
 class NetworkController;
 class SecondaryTableController;