Merge pull request #2140 from sunlong169/v1.x

No need to define the Mutex mutex_ as mutable there is no const method.

include/spdlog/sinks/base_sink.h

@@ -37,7 +37,7 @@ class base_sink : public sink
 protected:
  // sink formatter
  std::unique_ptr<spdlog::formatter> formatter_;
- mutable Mutex mutex_;
+ Mutex mutex_;
 
  virtual void sink_it_(const details::log_msg &msg) = 0;
  virtual void flush_() = 0;