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.

Fixes https://gitlab.freedesktop.org/xorg/xserver/issues/892



Reviewed-by: Aaron Plattner <aplattner@nvidia.com>
Reviewed-by: Alex Goins <agoins@nvidia.com>
(cherry picked from commit 0d8cc7bb)