output_configuration: Set wlr_output_head serial number

src/backend/x11.rs

@@ -94,6 +94,7 @@ impl X11State {
             subpixel: Subpixel::Unknown,
             make: "COSMIC".to_string(),
             model: name.clone(),
+            serial_number: "Unknown".to_string(),
         };
         let mode = Mode {
             size: (size.w as i32, size.h as i32).into(),