ci: add xorgproto dep to Arch build

Arch Linux maintainers are still figuring out whether they should ask
people to explicitely install it or make it a dependency of libxcb again
[1].  In the meantime, add it as an explicit dependency. I'll revert
this patch if they decide otherwise.

[1]: https://bugs.archlinux.org/task/64914

.builds/archlinux.yml

@@ -14,6 +14,7 @@ packages:
   - xcb-util-errors
   - xcb-util-image
   - xcb-util-wm
+  - xorgproto
 sources:
   - https://github.com/swaywm/wlroots
 tasks: