Try fixing build failure on CI

No idea why the code compiles on my system. This change is based on the
output on build.kde.org.

CMakeFiles/testXRandRScreens.dir/__/plugins/platforms/x11/standalone/screens_xrandr.cpp.o:
In function `KWin::XRandRScreens::event(xcb_generic_event_t*)':
/home/jenkins/workspace/Plasma kwin kf5-qt5 SUSEQt5.9/plugins/platforms/x11/standalone/screens_xrandr.cpp:210:
undefined reference to `KWin::Options::currentRefreshRate()'
...