Fix plan9 build

[ncw]

This fixes the build under plan9 - before this fix it was erroring with

$ GOOS=plan9 go build
# github.com/mattn/go-isatty
./isatty_plan9.go:11:30: cannot use fd (type uintptr) as type int in argument to syscall.Fd2path

Maybe we should add a compile all step to the travis tests?

[coveralls]

Coverage remained the same at 100.0% when pulling 785e92b on ncw:fix-plan9-build into 0e9ddb7 on mattn:master.

[mattn]

Thank you