xen: constify all instances of "struct attribute_group"

The functions these get passed to have been taking pointers to const
since at least 2.6.16.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
diff --git a/drivers/xen/xen-selfballoon.c b/drivers/xen/xen-selfballoon.c
index 767ff65..146c948 100644
--- a/drivers/xen/xen-selfballoon.c
+++ b/drivers/xen/xen-selfballoon.c
@@ -488,7 +488,7 @@
 	NULL
 };
 
-static struct attribute_group selfballoon_group = {
+static const struct attribute_group selfballoon_group = {
 	.name = "selfballoon",
 	.attrs = selfballoon_attrs
 };