[x11] Update X11 time stamp on Wayland

Assume that Xwayland's current X11 time stamp corresponds to the system
monotonic time. Unfortunately, we cannot make roundtrips to Xwayland and
we cannot query the time stamp asynchronously because it may introduce
regressions in the standalone X11 window manager.

Differential Revision: https://phabricator.kde.org/D29250