QtOPCUA build fails with 'no-gui'

agent:2020/01/14 06:58:11 build.go:220: make[2]: Entering directory '/home/qt/work/qt/qtopcua/examples/opcua'
agent:2020/01/14 06:58:11 build.go:220: cd x509/ && ( test -e Makefile || /home/qt/work/install/bin/qmake -o Makefile /home/qt/work/qt/qtopcua/examples/opcua/x509/x509.pro INSTALL_PREFIX=/home/qt/work/install ) && make -f Makefile
agent:2020/01/14 06:58:11 build.go:220: Project ERROR: Unknown module(s) in QT: gui
agent:2020/01/14 06:58:11 build.go:220: Makefile:46: recipe for target 'sub-x509-make_first' failed
agent:2020/01/14 06:58:11 build.go:220: make[2]: *** [sub-x509-make_first] Error 3

Build log: https://testresults.qt.io/logs/qt/qtopcua/dda82b4a5622bdd80a64f8f85bea71798a1861d7/LinuxUbuntu_18_04x86_64LinuxUbuntu_18_04x86_64GCCqtci-linux-Ubuntu-18.04-x86_64-93b842DisableTests/b82038800a3de538c8cfd159c5218ac234b4dc87/build_1578984895/log.txt.gz

Details: http://ci-files01-hki.intra.qt.io:8080/coin/integration/qt/tqtc-qt5/tasks/1578981652
Tested changes (refs/builds/qtci/tqtc/5.14/1578980939):
https://codereview.qt-project.org/c/qt/tqtc-qt5/+/286200 Merge remote-tracking branch 'origin/5.14' into tqtc/5.14
https://codereview.qt-project.org/c/qt/tqtc-qt5/+/286201 Update submodules on 'tqtc/5.14' in qt5

This is caused by f09dbf65e16dc53eb52555fdc26065ecef09cfc8 & should be fixed similarly like already done for mqtt: 61a68c296271d527899f64b5b38ed8fa46dc11dc