meson: Set .so version for xa like autotools does

Fixes: 0ba909f0
       ("meson: build gallium xa state tracker")
Signed-off-by: Dylan Baker <dylan.c.baker@intel.com>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
(cherry picked from commit 19dbed64)