localed: downgrade libxkbcommon to an optional runtime dependency

localed: downgrade libxkbcommon to an optional runtime dependency

Previously, libxkbcommon was a compile-time option. When enabled the localed
binary would strictly depend on it, thus pulling in libxkbcommon and its
dependencies, which are non-trivial in size.

With this change we dlopen() libxkbcommon when it is available instead. If the
library is available behaviour is as before. However, if it isn't the system is
considered "headless", i.e. without local hardware and all attempts to set the
local keyboard configuration will be refused.

This is useful for general-purpose distributions which want to support
"headless" (such as container systems) and "full" systems with the same build.