PM: sleep: Constify static struct attribute_group

The only usage of suspend_attr_group is to put its address in an
array of pointers to const attribute_group structs.

Make it const to allow the compiler to put it into read-only memory.
Signed-off-by: Rikard Falkeborn <rikard.falkeborn@gmail.com>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>

kernel/power/main.c

@@ -387,7 +387,7 @@ static struct attribute *suspend_attrs[] = {
 	NULL,
 };
 
-static struct attribute_group suspend_attr_group = {
+static const struct attribute_group suspend_attr_group = {
 	.name = "suspend_stats",
 	.attrs = suspend_attrs,
 };