powerpc/xmon: 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: Michael Ellerman <mpe@ellerman.id.au>
Cc: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Cc: Paul Mackerras <paulus@samba.org>
Cc: linuxppc-dev@lists.ozlabs.org
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Emil Velikov <emil.l.velikov@gmail.com>
Link: https://lore.kernel.org/r/20200513214351.2138580-6-emil.l.velikov@gmail.comSigned-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

arch/powerpc/xmon/xmon.c

@@ -3842,7 +3842,7 @@ static void sysrq_handle_xmon(int key)
 		xmon_init(0);
 }
 
-static struct sysrq_key_op sysrq_xmon_op = {
+static const struct sysrq_key_op sysrq_xmon_op = {
 	.handler =	sysrq_handle_xmon,
 	.help_msg =	"xmon(x)",
 	.action_msg =	"Entering xmon",