ci: Enable -Werror on the meson-i386 build.

I find warnings to be very disruptive to my workflow (using emacs's "go to
next error" feature), and I periodically have to go clean up other
people's drivers to get back to finding my own warnings in the noise.  I
know I'm not the only one doing something like this.

We don't want to enable -Werror by default in builds, since it means that
end users will have builds spuriously fail based on what compiler version
and opt flags they have compared to what the devs are using.  However, it
is quite easy to have CI ensure that we at least don't introduce warnings
on the compiler version that it uses.

For now I've just enabled it on meson-i386 to cover a bunch of Mesa core
and get us started on ratcheting up warnings-cleanliness in the tree,
without me having to fix up all the drivers at once.

Reviewed-by: Daniel Stone <daniels@collabora.com>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Reviewed-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Tested-by: Marge Bot <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3539>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/merge_requests/3539>

.gitlab-ci.yml

@@ -466,6 +466,7 @@ meson-i386:
       -D vulkan-overlay-layer=true
       -D llvm=false
       -D osmesa=classic
+      -D werror=true
 
 meson-mingw32-x86_64:
   extends: .meson-build