make docbook compilation optional
Summary:
khelpcenter isn't nearly as omnipresent as it was and redirecting the user
to the html render of the documentation online is a very common occurance.
in particular on Windows where I think we do not even ship the docbooks
at all.
Test Plan: cmake reports dep as optional
Reviewers: kfunk, sandsmark, adridg
Reviewed By: adridg
Subscribers: kde-utils-devel
Differential Revision: https://phabricator.kde.org/D8932