Fixed rolling file extension methods namespace regression

Author: nblumhardt

src/Serilog.Sinks.RollingFile/RollingFileLoggerConfigurationExtensions.cs

@@ -17,8 +17,9 @@
 using Serilog.Core;
 using Serilog.Events;
 using Serilog.Formatting.Display;
+using Serilog.Sinks.RollingFile;
 
-namespace Serilog.Sinks.RollingFile
+namespace Serilog
 {
     public static class RollingFileLoggerConfigurationExtensions
     {