update from DuongTD

include/robot_costmap_2d/preferred_layer.h

@@ -10,6 +10,12 @@ class PreferredLayer : public StaticLayer
 public:
   PreferredLayer();
   virtual ~PreferredLayer();
+
+  LayerType getType() const override
+  {
+      return LayerType::PREFERRED_LAYER;
+  }
+  
 private:
   unsigned char interpretValue(unsigned char value);
 };