Fail the build early if xmllint is not available

Summary:
xmllint is required later anyway, and kio_help has the check.
BUG: 376246

Test Plan: If xmllint is not available, compilation fails in the cmake phase.

Reviewers: winterz

Reviewed By: winterz

Subscribers: #frameworks, #documentation

Tags: #frameworks, #documentation

Differential Revision: https://phabricator.kde.org/D4558