CI: gtk4 builds
With the ubuntu-22.04 runner from #43598 we will have also gtk4 libraries available. Use them to build gtk4 parts of freeciv.
For enabling gtk4-client build on meson job, #44800 is needed.
Reply To cazfi
-> #44800 is a dependency for the attached patch.
With the ubuntu-22.04 runner from #43598 we will have also gtk4 libraries available. Use them to build gtk4 parts of freeciv.