Update null_sink.h

include/spdlog/sinks/null_sink.h

@@ -20,7 +20,7 @@ protected:
     void flush_() override {}
 };
 
-using null_sink_mt = null_sink<std::mutex>;
+using null_sink_mt = null_sink<details::null_mutex>;
 using null_sink_st = null_sink<details::null_mutex>;
 
 } // namespace sinks