backends/x11: Arm vsync monitor in EglX11Backend
This was originally on EglX11Backend::endFrame and got lost
as part of refactoring in 7358daa9, breaking nested kwin_wayland
rendering with GL.
Kai Uwe Broulik kai_uwe.broulik@mbition.io on behalf of MBition GmbH
https://github.com/mercedes-benz/daimler-foss/blob/master/PROVIDER_INFORMATION.md