Do not hard runtime depend on X11 in RuleBook

Summary:
The RuleBook is created during Workspace startup, so it's a required
component for the overall KWin session. It uses a KXMessages object which
means it has a hard X11 runtime dependency.

This change makes the dependency optional and creates the KXMessages once
X11 is available.

Test Plan: Compiles

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D7653