miext/sync: Fix ABI change on 32bit

The newly added field `initialized` increases the size of the structure
`_SyncObject` on 32bit, which is embedded in other structures, and
therefore can cause a crash of the Xserver when a client tries to create
a fence.

To avoid changing the size of the structure, and therefore breaking the
ABI, move the field back where it was initially but change its type to a
1 byte `char` instead of `Bool` which is defined as an integer.

Closes: xorg/xserver#892
Fixes: 8449c862 miext/sync: Fix needless ABI change
Suggested-by: Michel Dänzer <mdaenzer@redhat.com>
Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
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