Make zbus optional under "dbus" feature

src/backend/tty.rs

@@ -924,6 +924,7 @@ impl Tty {
     }
 
     pub fn suspend(&self) {
+        #[cfg(feature = "dbus")]
         if let Err(err) = suspend() {
             warn!("error suspending: {err:?}");
         }
@@ -1026,6 +1027,7 @@ fn refresh_interval(mode: DrmMode) -> Duration {
     Duration::from_nanos(refresh_interval)
 }
 
+#[cfg(feature = "dbus")]
 fn suspend() -> anyhow::Result<()> {
     let conn = zbus::blocking::Connection::system().context("error connecting to system bus")?;
     let manager = logind_zbus::manager::ManagerProxyBlocking::new(&conn)