gitlab-ci: Build CI with -Werror

We have no warnings now, and it would be great to keep that way (e.g.
!166 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>
2 jobs for gitlab-ci-werror in 1 minute and 12 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Container Prep
passed #254392
container_prep

00:00:41

 
  Build
passed #254393
build-native-meson

00:00:31