Restore "available" metadata on eio_windows

[talex5]

ocaml-ci currently only tests Eio on Linux because uring isn't installable on macos. And if eio_windows correctly stated that it only works on Windows, we wouldn't be able to test anywhere at all!

ocurrent/solver-service#72 will fix that, by testing only the compatible packages on each platform. Once it's in, uncomment the "#available" line in eio_windows.opam.template so that package has correct metadata (and doesn't need to be corrected manually on each release).