find qt5xml as dependency

Summary:
it's a public link target for the main library so we always need it found

seen used in

```
Analitza5Targets.cmake:  INTERFACE_LINK_LIBRARIES "Qt5::Core;Qt5::Xml"
```

Test Plan:
- app using the target wants to link Qt5::Xml but doesn't know what that means
- apply patch
- app now actually knows what Qt5::Xml is and builds properly

Reviewers: apol

Reviewed By: apol

Subscribers: kde-edu

Tags: #kde_edu

Differential Revision: https://phabricator.kde.org/D22655