MAINTAINERS: Add Philippe, Alex and Wainer to the Gitlab-CI section

Initially, I was the only one who was using Gitlab while most developers
had their git trees still on other systems, but that has changed nowadays.
There is now much more interest in the Gitlab-CI today, so it would be
good to have more than only one maintainer / reviewer for the gitlab-ci.yml
file. Alex, Wainer and Philippe kindly offered their help here, so let's
add them to the corresponding section in the MAINTAINERS file now.

Message-Id: <20200210155115.9371-1-thuth@redhat.com>
Reviewed-by: Alex Bennée <alex.bennee@linaro.org>
Reviewed-by: Philippe Mathieu-Daude <philmd@redhat.com>
Reviewed-by: Wainer dos Santos Moschetta <wainersm@redhat.com>
Signed-off-by: Thomas Huth <thuth@redhat.com>