gitlab-ci: Build CI with -Werror

We have no warnings now, and it would be great to keep that way (e.g. !166 (merged) which fixed a warning that should really have been a failure).

Enable -Werror builds in CI so we don't have any warnings creep in. This will result in container rebuilds which upgrade the toolchain and produce more warnings failing, until the code has been fixed to not warn.

-Werror should emphatically not be the default for non-CI builds, however. Doing this means that people trying to build older releases on newer/different toolchains will run into spurious failures. Werror is a tool for the development process, not something users should see: https://flameeyes.blog/2009/02/25/future-proof-your-code-dont-use-werror/

Signed-off-by: Daniel Stone daniels@collabora.com