miext/sync: Handle libxshmfence API change

libxshmfence had an unfortunate 'int32_t' type for the mapped fence.
That changed to exposing a 'struct shmfence' instead, which is nice
and opaque and offers fine type checking across the API.

This patch requires the newer version of the library and uses
the new interface type.

Signed-off-by: Keith Packard <keithp@keithp.com>
Reviewed-by: Julien Cristau <jcristau@debian.org>