message: Add missing StringUtils include to ListBox
diff: #include "../../FontManager.h"
  #include <NAS2D/Utility.h>
  +#include <NAS2D/MathUtils.h>
  +#include <NAS2D/StringUtils.h>
  #include <NAS2D/Renderer/Renderer.h>
  #include <NAS2D/Renderer/Point.h>
  -#include <NAS2D/MathUtils.h>
  #include <algorithm>
  #include <cmath>
  