fix missing include <string.h>

Can't compile vmap extraction tools without this include under Debian Jessie

src/game/vmap/WorldModel.cpp

@@ -25,6 +25,7 @@
 #include "WorldModel.h"
 #include "VMapDefinitions.h"
 #include "MapTree.h"
+#include <string.h>
 
 using G3D::Vector3;
 using G3D::Ray;