Summary:
The SyncManager is highly X11 dependent and doesn't make much sense on
Wayland. It should already be guarded by the check for
GL_EXT_x11_sync_object, but better be safe than sorry, so explicit check
for on X11.

Reviewers: #kwin, #plasma

Subscribers: plasma-devel, kwin

Tags: #kwin

Differential Revision: https://phabricator.kde.org/D7514