Add the remaining SPDX license markers and activate the Gitlab REUSE check

.gitlab-ci.yml

+# SPDX-FileCopyrightText: 2020-201 Laurent Montel <montel@kde.org>
+# SPDX-FileCopyrightText: 2021 Volker Krause <vkrause@kde.org>
+# SPDX-License-Identifier: CC0-1.0
+
 include:
   - https://invent.kde.org/sysadmin/ci-tooling/raw/master/invent/ci-before.yml
   - https://invent.kde.org/sysadmin/ci-tooling/raw/master/invent/ci-applications-linux.yml
@@ -28,3 +32,10 @@ build_clazy_clang_tidy:
   variables:
     PLATFORM: SUSEQt5.15
     BRANCH_GROUP: kf5-qt5
+
+reuse:
+  image:
+    name: fsfe/reuse:latest
+    entrypoint: [""]
+  script:
+    - reuse lint

CMakePresets.json.license

+# SPDX-FileCopyrightText: 2021 Laurent Montel <montel@kde.org>
+# SPDX-License-Identifier: BSD-3-Clause