[build] Fix compilation with GOOS=windows
[build] Fix compilation with GOOS=windows

Works around the fact that `syscall.Stdin` doesn't seem to be of the
same type (int) across Windows and other systems:

    $ GOOS=windows GOARCH=amd64 make
    script/build -o bin/hub
    # github.com/github/hub/github
    github/config.go:196:44: cannot use stdin (type syscall.Handle) as type int in argument to terminal.GetState
    github/config.go:205:19: cannot use stdin (type syscall.Handle) as type int in argument to terminal.Restore
    github/config.go:215:41: cannot use stdin (type syscall.Handle) as type int in argument to terminal.ReadPassword
    make: *** [bin/hub] Error 2

https://travis-ci.org/github/hub/jobs/587860665

github/hub,github/hub,github/hub,github/hub