alpha: constify sysrq_key_op

With earlier commits, the API no longer discards the const-ness of the
sysrq_key_op. As such we can add the notation.

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Jiri Slaby <jslaby@suse.com>
Cc: linux-kernel@vger.kernel.org
Cc: Richard Henderson <rth@twiddle.net>
Cc: Ivan Kokshaysky <ink@jurassic.park.msu.ru>
Cc: Matt Turner <mattst88@gmail.com>
Cc: linux-alpha@vger.kernel.org
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://lore.kernel.org/r/20200513214351.2138580-4-emil.l.velikov@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/alpha/kernel/setup.c

@@ -431,7 +431,7 @@ register_cpus(void)
 arch_initcall(register_cpus);
 
 #ifdef CONFIG_MAGIC_SYSRQ
-static struct sysrq_key_op srm_sysrq_reboot_op = {
+static const struct sysrq_key_op srm_sysrq_reboot_op = {
 	.handler	= machine_halt,
 	.help_msg       = "reboot(b)",
 	.action_msg     = "Resetting",