add systemd_service rust plugin (step 1a, no qml wiring yet)

plugin/build.rs

@@ -31,6 +31,7 @@ fn main() {
         "src/system_stats.rs",
         "src/cpu_service.rs",
         "src/modules_service.rs",
+        "src/systemd_service.rs",
         "src/theme_service.rs",
     ];
     if !bridge_files.iter().all(|p| manifest.join(p).exists()) {