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