openconnect: add missing dependency Qt5Xml
Summary: QDomDocument is imported from vpn/openconnect/openconnectauth.cpp
Test Plan: Build succeeds now.
Reviewers: Plasma, alexeymin
Reviewed By: alexeymin
Subscribers: alexeymin, plasma-devel
Tags: Plasma
Differential Revision: https://phabricator.kde.org/D16073