tango-icon-theme: add license

pkgs/data/icons/tango-icon-theme/default.nix

···
 
   postInstall = '''${gtk.out}/bin/gtk-update-icon-cache' "$out/share/icons/Tango" '';
 
-  meta = {
+  meta = with lib; {
     description = "A basic set of icons";
     homepage = "http://tango.freedesktop.org/Tango_Icon_Library";
-    platforms = lib.platforms.linux;
+    platforms = platforms.linux;
+    license = licenses.publicDomain;
   };
 }