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>
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 573c106..85c390d 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -562,6 +562,8 @@
     - kvm
   variables:
     CROSS: s390x
+    EXTRA_OPTION: >
+      -D werror=true
     GALLIUM_DRIVERS: "swrast"
   script:
     # For unknown reasons "too many" installed i386 libraries cause qemu to
@@ -576,6 +578,7 @@
     - meson-s390x
   variables:
     CROSS: ppc64el
+    EXTRA_OPTION: ""
     GALLIUM_DRIVERS: "nouveau,radeonsi,swrast,virgl"
     VULKAN_DRIVERS: "amd"