main: gate the restart import to unix

[obodnikov]

What does this PR do?

src/main/restart.rs is #![cfg(unix)]. A cfg-false module file is absent,
not empty, so on a non-unix target tuwunel::restart does not exist and the
unconditional import in main.rs is a hard error:

error[E0432]: unresolved import `tuwunel::restart`
 --> src/main/main.rs:3:47
  |
3 | use tuwunel::{Server, args, health::check, restart, runtime::Runtime};
  |                                            ^^^^^^^ no `restart` in the root

The call site is already gated:

#[cfg(unix)]
if server.server.restarting.load(Ordering::Acquire) {
    restart::restart();
}

so this only extends the same condition to the import that serves it. Ordering
follows for the same reason — its sole use is inside that block, so it is an
unused import on any non-unix target.

+#[cfg(unix)]
 use std::sync::atomic::Ordering;
 
-use tuwunel::{Server, args, health::check, restart, runtime::Runtime};
+#[cfg(unix)]
+use tuwunel::restart;
+use tuwunel::{Server, args, health::check, runtime::Runtime};

No behaviour change on unix. Same class of fix as #526.

On verification. CI builds Linux only, where none of this appears, so CI
cannot demonstrate the change. It was observed building v1.8.2 for
x86_64-pc-windows-msvc; the module-absence behaviour is reproducible with a
three-file crate on any host. cargo check and cargo clippy on the unix path
are clean before and after.

Checklist

• Code is formatted with nightly cargo fmt and satisfies clippy and
  rustc lints; any allowed lint is justified by an obvious reason or a
  comment.
• Complement compliance changes (new passes or new failures), if any,
  are noted in the description above. — none; no runtime path is touched.
• Config option changes were made in src/core/config/mod.rs doc
  comments and the regenerated tuwunel-example.toml is committed. — n/a
• User-facing changes are reflected in docs/. — n/a
• I agree that my changes may be licensed under the Apache-2.0 licence
  and my conduct is in line with the Contributor's Covenant and
  Tuwunel's Code of Conduct.