radicle-systemd: Guard `mod listen` for Unix

This crate does not build on Windows, because there is a Unix-only `use`
in `mod listen`. To get the crate to build, guard the module
appropriately.

crates/radicle-systemd/src/lib.rs

@@ -3,5 +3,5 @@
 #[cfg(all(feature = "journal", target_os = "linux"))]
 pub mod journal;
 
-#[cfg(feature = "listen")]
+#[cfg(all(feature = "listen", unix))]
 pub mod listen;