message: core: fix missing private access modifier
diff: @@ -211,6 +211,7 @@ public:
  void unregister_mavlink_command_handler(uint16_t cmd_id, const void* cookie);
  void unregister_all_mavlink_command_handlers(const void* cookie);
  +private:
  SystemImpl& _parent;
  void receive_command_int(const mavlink_message_t& message);
  