message: Fix compile warning of non-override decoration in gelf_appender
diff: @@ -28,7 +28,7 @@ namespace fc
  * In a single-threaded world with a boost::io_service that's not owned
  * by this library, ugly things are required. Tough.
  */
  - void initialize(boost::asio::io_service& io_service);
  + void initialize(boost::asio::io_service& io_service) override;
  virtual void log(const log_message& m) override;
  private:
  