zimreader: add license

See https://phabricator.wikimedia.org/source/openzim/browse/master/zimreader/COPYING

pkgs/tools/text/zimreader/default.nix

···
   meta = {
     description = "A tool to serve ZIM files using HTTP";
     homepage = http://git.wikimedia.org/log/openzim;
+    license = stdenv.lib.licenses.gpl2;
     maintainers = with stdenv.lib.maintainers; [ robbinch juliendehos ];
     platforms = [ "x86_64-linux" ];
   };