.gitlab-ci.yml: run pipeline in merge requests too

Hopefully this will allow us to run pipelines in regular branches but
also run it on merge requests on the parent project. This is needed
for users that are not verified by Gitlab.
https://docs.gitlab.com/ee/ci/pipelines/merge_request_pipelines.html#run-pipelines-in-the-parent-project

.gitlab-ci.yml

@@ -4,6 +4,12 @@ image: ubuntu:latest
 # XXX - add a deploy stage to publish man pages, docs, and coverage
 # reports
 
+workflow:
+  rules:
+    - if: $CI_PIPELINE_SOURCE == 'merge_request_event'
+    - if: $CI_COMMIT_TAG
+    - if: $CI_COMMIT_BRANCH
+
 stages:
   - build
   - test