os: fix Windows build from earlier readdir change
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
http://codereview.appspot.com/4535072

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