gitlab: don't run the DCO check on git master

The "only: branches" does not in fact exclude "master", so it must be
excluded explicitly.
Signed-off-by: NDaniel P. Berrangé <berrange@redhat.com>

.gitlab-ci.yml

@@ -228,3 +228,4 @@ dco:
     - branches
   except:
     - /^v.*-maint$/
+    - master