Check `SPDX-License-Identifier` in file headers.
Related to https://github.com/go-gitea/gitea/pull/21840
Co-authored-by: Jason Song <i@wolfogre.com>
Reviewed-on: https://gitea.com/gitea/gitea-vet/pulls/21
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Reviewed-by: Xinyu Zhou <i@sourcehut.net>
Co-authored-by: Jason Song <wolfogre@noreply.gitea.io>
Co-committed-by: Jason Song <wolfogre@noreply.gitea.io>