gitlab-ci: Enable failure on clang warings

Debian buster has mesa 0.49.0 that splits SCANBUILD on shlex:

    https://github.com/mesonbuild/meson/pull/4346
5 jobs for meson-clang in 3 minutes and 43 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
failed #7632
librem5
build-debian-clang

00:02:01

passed #7630
librem5
build-debian-gcc

00:02:55

39.3%
passed #7631
librem5
build-debian-gcc-static

00:02:00

passed #7634
librem5
build-debian-package

00:01:41

passed #7633
librem5
build-fedora-gcc

00:03:01

 
Name Stage Failure
failed
build-debian-clang Build
[0/1] /usr/bin/meson --internal scanbuild /builds/guido.gunther/libhandy /builds/guido.gunther/libhandy/_build /usr/bin/meson -Dexamples=true -Dglade_catalog=auto -Dgtk_doc=false -Dintrospection=auto -Dpackage_subdir= -Dprofiling=false -Dstatic=false -Dtests=true -Dvapi=true
Could not execute scan-build "scan-build --status-bugs"
FAILED: meson-scan-build
/usr/bin/meson --internal scanbuild /builds/guido.gunther/libhandy /builds/guido.gunther/libhandy/_build /usr/bin/meson -Dexamples=true -Dglade_catalog=auto -Dgtk_doc=false -Dintrospection=auto -Dpackage_subdir= -Dprofiling=false -Dstatic=false -Dtests=true -Dvapi=true
ninja: build stopped: subcommand failed.
Uploading artifacts...
WARNING: _build/meson-logs/scanbuild: no matching files
Uploading artifacts to coordinator... ok
id=7632 responseStatus=201 Created token=SdD2uVHv
ERROR: Job failed: exit code 1