meson: ensure that xmlpool_options.h is generated for gallium targets that need it

Fixes: 68076b87 "meson: build gallium vdpau state tracker"
Fixes: 22a817af "meson: build gallium xvmc state tracker"
Fixes: 5a785d51 "meson: build gallium va state tracker"
Fixes: 0ba909f0 "meson: build gallium xa state tracker"
Fixes: 1d36dc67 "meson: build gallium omx state tracker"
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
(cherry picked from commit 6fa923a6)