gdb/python: Fix --disable-tui build

As of 2023-02-13 "gdb/python: deallocate tui window factories at Python
shut down" (9ae4519da9), a TUI-less build fails with:

$src/gdb/python/py-tui.c: In function ‘void gdbpy_finalize_tui()’:
$src/gdb/python/py-tui.c:621:3: error: ‘gdbpy_tui_window_maker’ has not been declared
  621 |   gdbpy_tui_window_maker::invalidate_all ();
      |   ^~~~~~~~~~~~~~~~~~~~~~

Since gdbpy_tui_window_maker is only defined under #ifdef TUI, add an
#ifdef guard in gdbpy_finalize_tui as well.

gdb/python/py-tui.c

@@ -618,5 +618,7 @@ gdbpy_initialize_tui ()
 void
 gdbpy_finalize_tui ()
 {
+#ifdef TUI
   gdbpy_tui_window_maker::invalidate_all ();
+#endif	/* TUI */
 }