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.
3 jobs for misync-abi in 6 minutes and 44 seconds (queued for 11 seconds)
Status Job ID Name Coverage
  Docker Image
passed #523113
debian-testing

00:00:32

 
  Build And Test
passed #523114
autotools-build-and-test

00:06:11

passed #523115
meson-build-and-test

00:01:56