gtk3: hacky fpicker button removal needs updating

because the gtk3 fpicker puts them in the new header bar
are now

Change-Id: I78334476b42366ffdc64da9460eb017c8bc61622
(cherry picked from commit 34059269)

vcl/unx/gtk/fpicker/SalGtkFilePicker.cxx

@@ -425,7 +425,7 @@ dialog_remove_buttons( GtkDialog *pDialog )
     GtkWidget *pActionArea;
 
 #if GTK_CHECK_VERSION(3,0,0)
-    pActionArea = gtk_dialog_get_action_area( pDialog );
+    pActionArea = gtk_dialog_get_header_bar(pDialog);
 #else
     pActionArea = pDialog->action_area;
 #endif