[rpms/perl-Mail-DKIM] PR #4: Update license to SPDX format

[Jitka Plesnikova]

jplesnik merged a pull-request against the project: `perl-Mail-DKIM` that you are
following.

Merged pull-request:

``
Update license to SPDX format
``

https://src.fedoraproject.org/rpms/perl-Mail-DKIM/pull-request/4