pdf-quench: fix "Namespace Gtk not available"

pdf-quench previously failed at runtime with "ValueError: Namespace Gtk
not available". This moves `gobject-introspection` to nativeBuildInputs,
which fixes the error.

pkgs/applications/misc/pdf-quench/default.nix

···
-{ lib, fetchFromGitHub, pkgs, python3, wrapGAppsHook}:
+{ lib, fetchFromGitHub, pkgs, python3, wrapGAppsHook, gobject-introspection }:
 
 python3.pkgs.buildPythonApplication {
   pname = "pdf-quench";
···
     sha256 = "1rp9rlwr6rarcsxygv5x2c5psgwl6r69k0lsgribgyyla9cf2m7n";
   };
 
-  nativeBuildInputs = [ wrapGAppsHook ];
+  nativeBuildInputs = [ wrapGAppsHook gobject-introspection ];
   buildInputs = with pkgs; [
     gtk3
-    gobject-introspection
     goocanvas2
     poppler_gi
   ];