miext/sync: Fix ABI for 1.20
The initialized field was added in:
commit 82f01ad7869e3f2be51e41a8246dab5982bbc36a Author: Alex Goins <email@example.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.