Modified sinks to be templates with Mutex param to support single threaded sinks

include/c11log/sinks/null_sink.h

@@ -1,12 +1,12 @@
 #pragma once
 
-#include <mutex>
-#include <memory>
 #include "base_sink.h"
 
 namespace c11log {
 namespace sinks {
-class null_sink : public base_sink
+
+template <class Mutex>
+class null_sink : public base_sink<Mutex>
 {
 protected:
     void _sink_it(const details::log_msg&) override