ci: Enable -Werror in all build jobs
While not all jobs are fully warning-clean yet, we can enable
-Werror in general and just allow the existing types of warnings as exceptions with
-Wno-error=[...]. This way, new warnings of all other types will be prevented from entering the code base.
Once all warnings of a certain type have been eliminated in a job, the exception for that type can be dropped from that job. This provides a realistic path to a fully warning-clean CI build in the future.
Pipeline with these changes: https://gitlab.freedesktop.org/daenzer/mesa/-/pipelines/300679