[PATCH wireguard-go] tun: make operateonfd.go build tags more specific

[Tobias Klauser]

On 2021-09-23 at 17:54:59 +0200, Jason A. Donenfeld <Jason at zx2c4.com> wrote:
> I've applied this. Thanks for the patch. Though I do wonder: it's
> functionally the same as before, right? What instances lead to this
> being an issue? Or did this way just seem more correct?

Thanks. Functionally it's the same, yes. I noticed this when running
staticcheck against wireguard-go on Linux and it reported
(*NativeTun).operateOnFd as being defined but not used. So it just
seemed more correct to only define it for platforms that actually use
it.