localed: validate xkb keymaps

Introduce a new optional dependency on libxkbcommon for systemd-localed.
Whenever the x11 keymap settings are changed, use libxkbcommon to compile
the keymap. If the compilation fails, print a warning so users will get
notified.

On compilation failure, we still update the keymap settings for now. This
patch just introduces the xkbcommon infrastructure to have keymap
validation in place. We can later decide if/how we want to enforce this.