systemd: Empty crate when target OS is not Linux — heartwood

This is a quick fix for build errors on macOS.

 version = "0.10.0"
 rust-version.workspace = true
 [dependencies]
 [target.'cfg(target_os = "linux")'.dependencies]
 log = { workspace = true, optional = true }
 systemd-journal-logger = { version = "2.2.2", optional = true }
 [features]
 default = ["journal", "listen"]
 journal = ["dep:log", "dep:systemd-journal-logger"]
 listen = []
 \ No newline at end of file
 listen = []

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