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