fix model diagnostic tool

modules/dnn/include/opencv2/dnn/layer.hpp

@@ -66,6 +66,9 @@ public:
     //! Unregisters registered layer with specified type name. Thread-safe.
     static void unregisterLayer(const String &type);
 
+    //! Check if layer is registered.
+    static bool isLayerRegistered(const std::string& type);
+
     /** @brief Creates instance of registered layer.
      *  @param type type name of creating layer.
      *  @param params parameters which will be used for layer initialization.