Add SPDX license information for flags data files

.reuse/dep5

+Format: https://www.debian.org/doc/packaging-manuals/copyright-format/1.0/
+Upstream-Name: Marble
+Upstream-Contact: Torsten Rahn <rahn@kde.org>
+Source: https://invent.kde.org/education/marble/
+
+Files: data/flags/*
+Copyright: none
+License: CC0-1.0
+