Fix code to support VS2013

include/c11log/logger.h

@@ -30,8 +30,10 @@ public:
         _logger_name(name),
         _formatter(new formatters::default_formatter()),
         _sinks(),
-        _mutex(),
-        _atomic_level(level::INFO) {
+        _mutex()
+    {
+        //Seems that vs2013 doesnt support atomic member initialization in ctor, so its done here
+        _atomic_level = level::INFO;
     }
 
     ~logger() = default;