ci: fix .gitlab-ci.yml syntax errors

.gitlab-ci.yml

 include:
   - project: 'quiqqer/semantic-release'
     file: '/.gitlab-ci.yml'
+stages:
+    - validate
+    - release
 validate_merge_request_source_branch:
+  stage: validate
   script:
     - echo "Job runs to validate merge request source branch"
   rules: