gitlab-ci: Enable -Werror in the meson build job

There are no warnings left, so let's prevent new ones from creeping
back in.
3 jobs for gitlab-ci-meson-werror in 6 minutes and 53 seconds
Status Job ID Name Coverage
  Docker Image
passed #1485512
debian-testing

00:00:30

 
  Build And Test
passed #1485513
autotools-build-and-test

00:06:23

passed #1485514
meson-build-and-test

00:02:10