Fix configure when xmllint is not present

Summary:
The breeze-validate-svg target is only generated when xmllint is found.
breeze-generate-24px-versions cannot be added as a dependency, unless
the target exists.

Reviewers: kossebau

Reviewed By: kossebau

Subscribers: ngraham, kde-frameworks-devel

Tags: Frameworks

Differential Revision: https://phabricator.kde.org/D28407