gitlab-ci: Enable -Werror in `meson-s390x` job

It's warning-clean.

v2:
* Prevent -Werror from being enabled in `meson-ppc64le` job as well

Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5185>
1 file changed