message: buffer filter: add static declaration macro
  Commit Message: buffer filter - add static declaration macro
  Additional Description: needed for static libraries using envoy.
  Risk Level: low
  Testing: n/a
diff: @@ -30,6 +30,8 @@ private:
  Server::Configuration::ServerFactoryContext&, ProtobufMessage::ValidationVisitor&) override;
  };
  +DECLARE_FACTORY(BufferFilterFactory);
  +
  } // namespace BufferFilter
  } // namespace HttpFilters
  } // namespace Extensions
  