[kwin] Work around broken xcb_sync_create_alarm

The xcb sync protocol is incorrectly defined (see [1]) which results in
xcb_sync_create_alarm not creating a valid alarm. To work around this
issue we only create the alarm without setting the int64 values. For
those we use the XLib XSyncChangeAlarm call after we verified that the
alarm got created. This unfortunately reintroduces linking against
libxext. But at least resizing works again.

[1] http://lists.freedesktop.org/archives/xcb/2013-June/008375.html