fix compile failure on windows

```
lucky(~/src/github.com/juju/utils/symlink) % env GOOS=linux go list -f '{{ .GoFiles }}'                                                               
[symlink.go symlink_posix.go]
lucky(~/src/github.com/juju/utils/symlink) % env GOOS=windows go list -f '{{ .GoFiles }}'                                                             
[symlink.go symlink_windows.go zsymlink_windows_amd64.go]
```