Don't use "with licenses;" for single licences

And don't use square brackets on such lines.

pkgs/tools/backup/bacula/default.nix

@@ -25,7 +25,7 @@ stdenv.mkDerivation rec {
   meta = with stdenv.lib; {
     description = "Enterprise ready, Network Backup Tool";
     homepage    = http://bacula.org/;
-    license     = with licenses; gpl2;
+    license     = licenses.gpl2;
     maintainers = with maintainers; [ iElectric lovek323 ];
     platforms   = platforms.all;
   };