When we depend on Qt Svg, we probably also want to be able to
use the image formats plugin, which also depends on Qt Xml. To
make this possible, we add a default dependency on libQt5Xml.

Task-number: QTBUG-34586
Change-Id: I59fa54eabb2e4f71d6b89cc1b04b36b704154a9b
Reviewed-by: Christian Stromme <christian.stromme@digia.com>