enforce SPDX headers by using reuse in CI pipeline

.gitlab-ci.yml

@@ -1,6 +1,13 @@
 # SPDX-FileCopyrightText: 2023 Tobias Schmidl
 # SPDX-License-Identifier: CC0-1.0
 
+reuse:
+  image:
+    name: fsfe/reuse:latest
+    entrypoint: [""]
+  script:
+    - reuse lint
+
 docker-build:
   image: docker:latest
   stage: build