localed: print a custom message if libxkbcommon.so is unvailable

localed: print a custom message if libxkbcommon.so is unvailable

Follow-up for 82c2095a5e407bcf041dc7bde84791deec95ff9c.

> I feel like the logging here may be a bit confusing on the new path.
> Previously you did get a message that explained what was going on. Now you get
> an info message that the layout could not be compiled, and … that's all. I can
> imagine this being a confusing red herring if someone was trying to debug a
> problem and saw this message. Perhaps we should log something else instead/as
> well, on the case where libxkbcommon isn't present, to say that's what we're
> logging about and it just means we can't validate the configuration, not that
> it's definitely invalid?