.gitlab-ci.yml: include pipelines for merge requests

This template is offered by GitLab to enable pipelines for merge
requests, a safety feature that avoids breaking the default branch by
merging things that passed in the past.

Reference: https://docs.gitlab.com/ee/ci/pipelines/merge_request_pipelines.html


Signed-off-by: Zygmunt Krynicki <zygmunt.krynicki@huawei.com>