systemd: Require Linux for journal module

The dependency `systemd-journal-logger` does not build on macOS, and
there are no tests for other Unixes. Also, systemd is most common on
Linux.

To avoid compilation errors on non-Linux platforms, only depend on the
crate when building on Linux.

Co-authored-by: Lorenz Leutgeb <lorenz.leutgeb@radicle.xyz>

crates/radicle-systemd/Cargo.toml

@@ -10,6 +10,8 @@ rust-version.workspace = true
 
 [dependencies]
 log = { workspace = true, optional = true }
+
+[target.'cfg(target_os = "linux")'.dependencies]
 systemd-journal-logger = { version = "2.2.2", optional = true }
 
 [features]

crates/radicle-systemd/src/lib.rs

@@ -1,6 +1,6 @@
 //! Library for interaction with systemd, specialized for Radicle.
 
-#[cfg(feature = "journal")]
+#[cfg(all(feature = "journal", target_os = "linux"))]
 pub mod journal;
 
 #[cfg(feature = "listen")]