KWin::updateXTime only delegates into the platform API where the method
is a no-op. The actual implementation is moved into the X11 standalone
platform as it uses QX11Info which is non functional except on the X11
standalone platform.

This change exposes a problem with timestamp handling: on Wayland the
X11 timestamp does not get updated at all, causing e.g. window sync not
work correctly (c.f. bug 374881). We cannot implement the updating in the
same way as QX11Info/Qt xcb platform does it as that would introduce a
blocking roundtrip to XWayland which is dangerous.

As a side-effect this change removes linking to Qt5::X11Extras in kwin
core as it's no longer needed.