Migrate to SPDX license (temporary ids)

This is part of https://fedoraproject.org/wiki/Changes/SPDX_Licenses_Phase_4

Note that this is an SPDX identifier, but it still needs to be migrated to the proper identifiers in the fedora-license-data set.
Please follow https://docs.fedoraproject.org/en-US/legal/update-existing-packages/#_public_domain

If there will be no comments in two weeks, I will merge this.

xmlpull.spec

@@ -3,7 +3,8 @@ Version:        1.2.0
 Release:        10%{?dist}
 Summary:        XML Pull Parsing API
 
-License:        Public Domain
+# Automatically converted from old format: Public Domain - needs further work
+License:        LicenseRef-Callaway-Public-Domain
 URL:            https://github.com/xmlpull-xpp3/%{name}-xpp3
 Source0:        %{url}/archive/%{name}-xpp3-parent-%{version}.tar.gz
 BuildArch:      noarch