Fix regression in System.FilePath.Windows.normalise

W.normalise "//." would return "\\\\.\\" instead of
"\\\\.".

This is a regression since the 1.4.100.0 rewrite.

Fixes #187

Author: hasufell

System/FilePath/Internal.hs

@@ -963,7 +963,7 @@ normalise filepath =
            then singleton _period
            else joinDrive d p
 
-    addPathSeparator = isDirPath filepath
+    addPathSeparator = isDirPath pth
       && not (hasTrailingPathSeparator result)
       && not (isRelativeDrive drv)