systemd: allow action field to be unread in demo_mode

This fixes a new cargo clippy error that complains about the `action`
field never being read in demo_mode:

    $ cargo clippy --features=demo_mode --no-default-features
    error: field `action` is never read
      --> src/dbus/systemd.rs:55:9
       |
    54 | pub struct Service {
       |            ------- field in this struct
    55 |     pub action: Arc<Topic<ServiceAction>>,
       |         ^^^^^^
       |

Signed-off-by: Leonard Göhrs <[email protected]>

Author: hnez

src/dbus/rauc.rs

@@ -102,6 +102,7 @@ pub struct Rauc {
     pub operation: Arc<Topic<String>>,
     pub progress: Arc<Topic<Progress>>,
     pub slot_status: Arc<Topic<Arc<SlotStatus>>>,
+    #[cfg_attr(feature = "demo_mode", allow(dead_code))]
     pub primary: Arc<Topic<String>>,
     pub last_error: Arc<Topic<String>>,
     pub install: Arc<Topic<String>>,