udev: Fetch seat-id via backend instead of laucher

Configurations where we won't have a MetaLauncher will be added, so
avoid using its API directly.

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1689>

src/backends/native/meta-backend-native.c

@@ -314,6 +314,12 @@ meta_backend_native_lock_layout_group (MetaBackend *backend,
   meta_backend_notify_keymap_layout_group_changed (backend, idx);
 }
 
+const char *
+meta_backend_native_get_seat_id (MetaBackendNative *backend_native)
+{
+  return meta_launcher_get_seat_id (backend_native->launcher);
+}
+
 static void
 meta_backend_native_set_pointer_constraint (MetaBackend           *backend,
                                             MetaPointerConstraint *constraint)