Fix build with directory-1.2.6.2.

It started exporting a isSymbolicLink which supports windows. But,
git-annex does no use symlinks on windows yet and this conflicts with the
function by the same name from unix-compat, so hide it.

Build/NullSoftInstaller.hs

@@ -21,7 +21,7 @@
 {-# LANGUAGE OverloadedStrings, FlexibleContexts #-}
 
 import Development.NSIS
-import System.Directory
+import System.Directory hiding (isSymbolicLink)
 import System.FilePath
 import Control.Monad
 import Control.Applicative