os: fix Windows build from earlier readdir change

It worked originally, until I renamed a variable. :/

dir_windows.go:11: undefined: e
http://godashboard.appspot.com/log/092397f2ac7a1e6e812dc6bebc65b40b02368700a119343d5cee5e2e89e0fde5

R=golang-dev, r
CC=golang-dev
https://golang.org/cl/4535072

src/pkg/os/dir_windows.go

@@ -8,7 +8,7 @@ func (file *File) Readdirnames(n int) (names []string, err Error) {
 	fis, err := file.Readdir(n)
 	// If n > 0 and we get an error, we return now.
 	// If n < 0, we return whatever we got + any error.
-	if n > 0 && e != nil {
+	if n > 0 && err != nil {
 		return nil, err
 	}
 	names = make([]string, len(fis))