Add SPDX license information for osm-bitmaps data files

.reuse/dep5

@@ -7,3 +7,7 @@ Files: data/flags/*
 Copyright: none
 License: CC0-1.0
 
+Files: data/bitmaps/osmcarto/*/*
+Copyright: Andy Allan <andy@gravitystorm.co.uk> and contributors, based on original cartography by Steve Chilton and others.
+License: CC0-1.0
+

data/bitmaps/osmcarto/LICENSE.txt

-OpenStreetMap Carto: A global map style for OpenStreetMap maps.
-
-Created by Andy Allan <andy@gravitystorm.co.uk> and contributors.
-Based on original cartography by Steve Chilton and others.
-
-This software and associated documentation files (the "Software") is
-released under the CC0 Public Domain Dedication, version 1.0, as
-published by Creative Commons. To the extent possible under law, the
-author(s) have dedicated all copyright and related and neighboring
-rights to the Software to the public domain worldwide. The Software is
-distributed WITHOUT ANY WARRANTY.
-
-If you did not receive a copy of the CC0 Public Domain Dedication
-along with the Software, see
-<http://creativecommons.org/publicdomain/zero/1.0/>
\ No newline at end of file