classes declared as final should not define protected fields

include/spdlog/async_logger.h

@@ -51,13 +51,12 @@ public:
 
     std::shared_ptr<logger> clone(std::string new_name) override;
 
-protected:
+private:
     void sink_it_(const details::log_msg &msg) override;
     void flush_() override;
     void backend_sink_it_(const details::log_msg &incoming_log_msg);
     void backend_flush_();
 
-private:
     std::weak_ptr<details::thread_pool> thread_pool_;
     async_overflow_policy overflow_policy_;
 };