Let unix_socket example compile but not run on Windows

[Aankhen]

It wasn’t possible to run the tests on Windows without this change, as the examples have to be compiled too:

> cargo test -- form_fields
   Compiling warp v0.3.1 (D:\Media\src\warp)
error[E0432]: unresolved import `tokio::net::UnixListener`
 --> examples\unix_socket.rs:3:5
  |
3 | use tokio::net::UnixListener;
  |     ^^^^^^^^^^^^------------
  |     |           |
  |     |           help: a similar name exists in the module: `TcpListener`
  |     no `UnixListener` in `net`

error[E0432]: unresolved import `tokio_stream::wrappers::UnixListenerStream`
 --> examples\unix_socket.rs:4:5
  |
4 | use tokio_stream::wrappers::UnixListenerStream;
  |     ^^^^^^^^^^^^^^^^^^^^^^^^------------------
  |     |                       |
  |     |                       help: a similar name exists in the module: `TcpListenerStream`
  |     no `UnixListenerStream` in `wrappers`

error: aborting due to 2 previous errors

For more information about this error, try `rustc --explain E0432`.
error: could not compile `warp`

To learn more, run the command again with --verbose.
warning: build failed, waiting for other jobs to finish...
error: build failed

With this change:

> cargo test -- form_fields
   Compiling warp v0.3.1 (D:\Media\src\warp)
    Finished test [unoptimized + debuginfo] target(s) in 42.24s
     Running unittests (target\debug\deps\warp-9e6997018269ce07.exe)
(test output elided)

I made it so the example panics on Windows, and I used Docker to verify I’m still able to make requests over the socket on Linux as well.

[jxs]

thanks! 💯

[Aankhen]

Thanks for the merge (and the tweaks)!