adtool: mark broken

Broke when updating OpenLDAP>2.5 and has not seen a change since 2017,
which is at this point 5y in the past.

I think it's safe to say that this tool deserves to being marked broken.

pkgs/tools/admin/adtool/default.nix

···
     homepage = "https://gp2x.org/adtool";
     license = licenses.gpl2;
     maintainers = with maintainers; [ peterhoeg ];
+    broken = true; # does not link against recent libldap versions and unmaintained since 2017
   };
 }