ci: Enable Werror on meson-arm64-build-test.

I found that it was warnings-clean already, let's make sure it stays that
way.

Reviewed-by: Rob Clark <robdclark@chromium.org>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7224>
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 9fe1d36..b98236c 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -779,6 +779,7 @@
     VULKAN_DRIVERS: "amd"
     EXTRA_OPTION: >
       -Dtools=panfrost
+      -D werror=true
   script:
     - .gitlab-ci/meson-build.sh