Clarify database license

It was intended to be under the GNU GPL v2 or later, as per the license fields as visible in Debian and Fedora packages.

http://changelogs.ubuntu.com/changelogs/pool/main/s/shared-mime-info/shared-mime-info_2.0-1/copyright

https://metadata.ftp-master.debian.org/changelogs//main/s/shared-mime-info/shared-mime-info_1.10-1_copyright

https://src.fedoraproject.org/rpms/shared-mime-info/blob/rawhide/f/shared-mime-info.spec#_5