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

It's warning-clean.

Reviewed-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5730>
diff --git a/.gitlab-ci.yml b/.gitlab-ci.yml
index 1284341..be43e51 100644
--- a/.gitlab-ci.yml
+++ b/.gitlab-ci.yml
@@ -558,6 +558,7 @@
     EXTRA_OPTION: >
       -D osmesa=classic
       -D tools=all
+      -D werror=true
 
 .meson-cross:
   extends: