Windows build

👋

this looks like a great tool! Surely, it's probably mostly used on Linux, but is there a reason (apart being not in focus) that it doesn't build on Windows? Walking across the devices and the comparison of the platform-dependent fields might need another implementation. Is this desirable, or are there insurmountable hurdles?

```
%GOPATH%\src\github.com\google\fswalker (master -> origin)
λ go build
# github.com/google/fswalker
.\walker.go:129:30: undefined: syscall.Stat_t
.\walker.go:251:36: undefined: syscall.Stat_t

%GOPATH%\src\github.com\google\fswalker (master -> origin)
λ go version
go version go1.12.5 windows/amd64
```