Update null_sink.h

include/spdlog/sinks/null_sink.h

@@ -27,7 +27,7 @@ protected:
 
 };
 typedef null_sink<details::null_mutex> null_sink_st;
-typedef null_sink<std::mutex> null_sink_mt;
+typedef null_sink<details::null_mutex> null_sink_mt;
 
 }
 }