When using the Meson build system, miext/sync would be build only for
dri3.
As a result, when building with Meson without DRI3 enabled, Xwayland
would fail to link because `miSyncShmScreenInit()` is nowhere to be
found.
Make sure to build miext/sync for either DRI3 or Xwayland.
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>