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.

Signed-off-by: Victoria Fischer <victoria.fischer@mbition.io>
Tested-by: Merge Service <!2458>
Part-of: <!2458>


(cherry picked from commit d143f6e3)