iommu: Make kobj_type structure constant

Since commit ee6d3dd4 ("driver core: make kobj_type constant.")
the driver core allows the usage of const struct kobj_type.

Take advantage of this to constify the structure definition to prevent
modification at runtime.
Signed-off-by: Thomas Weißschuh <linux@weissschuh.net>
Link: https://lore.kernel.org/r/20230214-kobj_type-iommu-v1-1-e7392834b9d0@weissschuh.netSigned-off-by: Joerg Roedel <jroedel@suse.de>

drivers/iommu/iommu.c

@@ -739,7 +739,7 @@ static void iommu_group_release(struct kobject *kobj)
 	kfree(group);
 }
 
-static struct kobj_type iommu_group_ktype = {
+static const struct kobj_type iommu_group_ktype = {
 	.sysfs_ops = &iommu_group_sysfs_ops,
 	.release = iommu_group_release,
 };