pdftk: fix build

Fixes #33509.

pkgs/tools/typesetting/pdftk/default.nix

···
     sha256 = "1hdq6zm2dx2f9h7bjrp6a1hfa1ywgkwydp14i2sszjiszljnm3qi";
   };
 
-  buildInputs = [ gcj unzip ];
+  nativeBuildInputs = [ gcj unzip ];
 
   hardeningDisable = [ "fortify" "format" ];