Add extension_language_ops::shutdown

Right now, Python is shut down via a final cleanup.  However, it seems
to me that it is better for extension languages to be shut down
explicitly, after all the ordinary final cleanups are run.  The main
reason for this is that a subsequent patch adds another case like
finalize_values; and rather than add a series of workarounds for
Python shutdown, it seemed better to let these be done via final
cleanups, and then have Python shutdown itself be the special case.

gdb/guile/guile.c

@@ -115,6 +115,7 @@ static const struct extension_language_ops guile_extension_ops =
 {
   gdbscm_initialize,
   gdbscm_initialized,
+  nullptr,
 
   gdbscm_eval_from_control_command,