gitlab: Extract DCO/style check jobs to static_checks.yml

Extract the DCO / checkpatch jobs to a new file (static_checks.yml)
to be able to run them without having to run all the jobs included
in the default .gitlab-ci.yml, which are mainly useful for the
mainstream CI.

Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Willian Rampazzo <willianr@redhat.com>
Message-Id: <20210519185504.2198573-6-f4bug@amsat.org>
Signed-off-by: Thomas Huth <thuth@redhat.com>