onion-tunnel: add #[cfg]s to make crate build on macOS
- The netlink stuff is Linux-specific, so that now only compiles on desktop Linux (which is good anyway, since that was at best a no-op on Android).
- smoltcp also doesn't let us use its
TunTapInterface
either, which makes sense; I guess the iOS/macOS crate to make this work will have to make a fd from a utun device manually.