buildroot

.gitlab-ci.yml: add check-DEVELOPERS test

This verifies that the DEVELOPERS file doesn't contain errors. It
just runs parse_developers from getdeveloperlib and errors out if
that produces any output.

Signed-off-by: Arnout Vandecappelle (Essensium/Mind) <arnout@mind.be>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>