Only report DocBookXML as found if it was actually found
Summary: Otherwise we generate files with paths such as "searched_dir-NOTFOUND/docbookx.dtd"
Reviewers: Frameworks, ltoscano
Reviewed By: ltoscano
Subscribers: kde-frameworks-devel, kde-doc-english
Tags: Frameworks, Documentation
Differential Revision: https://phabricator.kde.org/D19224