'struct shutdown_handler' is not modified in this driver.

Constifying this structure moves some data to a read-only section, so
increase overall security.

On a x86_64, with allmodconfig:
Before:
======
   text	   data	    bss	    dec	    hex	filename
   7043	    788	      8	   7839	   1e9f	drivers/xen/manage.o

After:
=====
   text	   data	    bss	    dec	    hex	filename
   7164	    676	      8	   7848	   1ea8	drivers/xen/manage.o
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Reviewed-by: Juergen Gross <jgross@suse.com>
Link: https://lore.kernel.org/r/ca1e75f66aed43191cf608de6593c7d6db9148f1.1719134768.git.christophe.jaillet@wanadoo.frSigned-off-by: Juergen Gross <jgross@suse.com>