wayland: dispatcher: warn on wl_compositor event

Be consistent with other object events.

src/wayland/dispatcher.rs

@@ -142,6 +142,7 @@ impl Dispatch<wl_compositor::WlCompositor, ()> for Dispatcher {
              _conn: &Connection,
              _qh: &QueueHandle<Dispatcher>)
     {
+        eprintln!("warn: unknown wl_compositor event emitted");
     }
 }