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)
3 jobs for misync-abi-for-1.20 in 8 minutes and 19 seconds (queued for 46 seconds)
Status Job ID Name Coverage
  Docker Image
passed #523136
debian-testing

00:01:00

 
  Build And Test
passed #523137
autotools-build-and-test

00:06:38

passed #523138
meson-build-and-test

00:01:32