gdk-pixbuf: remove obsolete gobject-introspection from buildInputs

Since #200337, it is propagated from nativeBuildInputs.

pkgs/development/libraries/gdk-pixbuf/default.nix

···
     fixDarwinDylibNames
   ];
 
-  buildInputs = [ gobject-introspection ];
-
   propagatedBuildInputs = [
     glib
     libtiff