miext/sync: Make struct _SyncObject::initialized fully ABI compatible
With a 32-bit build, putting the initialized field at the end of the struct bumped the struct size from 20 bytes to 24, changing the layout of other structs embedding struct _SyncObject. While this would be acceptable on master, it caused crashes with 1.20.
Making the initialized field a char and putting it in the hole before the beingDestroyed field restores the 32-bit ABI as well.