k9copy, kpmcore: Mark as broken

See: https://github.com/NixOS/nixpkgs/pull/63260#pullrequestreview-251601561

The build for kpmcore is only broken when the Qt version is older than
5.12.

pkgs/development/libraries/kpmcore/default.nix

@@ -28,6 +28,8 @@ in stdenv.mkDerivation rec {
   enableParallelBuilding = true;
 
   meta = with stdenv.lib; {
-   maintainers = with lib.maintainers; [ peterhoeg ];
+    maintainers = with lib.maintainers; [ peterhoeg ];
+    # The build requires at least Qt 5.12:
+    broken = lib.versionOlder qtbase.version "5.12.0";
   };
 }