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

It's warning-clean.

Reviewed-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5799>
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 6635705..0b806a7 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -555,6 +555,7 @@
     EXTRA_OPTION: >
       -D osmesa=gallium
       -D tools=all
+      -D werror=true
   script:
     - .gitlab-ci/meson-build.sh
     - .gitlab-ci/run-shader-db.sh