miext/sync: Fix needless ABI change

The initialized field was added in:

    commit 82f01ad7
    Author: Alex Goins <agoins@nvidia.com>
    Date:   Wed Apr 10 13:48:02 2019 -0500

        xsync: Add resource inside of SyncCreate, export SyncCreate

But it added this field not at the end of SyncObject. It may not have
been _usefully_ possible to create those from another extension prior to
that commit, but that's still an ABI-incompatible change.

(cherry picked from commit 194ba387)