cleaned common.h and moved some code around

include/spdlog/sinks/file_sinks.h

@@ -20,7 +20,7 @@
 namespace spdlog
 {
 namespace sinks
-{
+{	
 /*
 * Trivial file sink with single file as target
 */
@@ -108,6 +108,7 @@ private:
 
     void _rotate()
     {
+		using details::os::filename_to_str;
         _file_helper.close();
         for (auto i = _max_files; i > 0; --i)
         {