samples/kobject: make attribute_group const

The attr_group structures are allocated once and never modified at
runtime. Also to match the const‑qualified parameter of
sysfs_create_group().

Signed-off-by: Meng Shao Liu <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Greg Kroah-Hartman <[e-mail]>

samples/kobject/kobject-example.c

···
  * created for the attributes with the directory being the name of the
  * attribute group.
  */
-static struct attribute_group attr_group = {
+static const struct attribute_group attr_group = {
 	.attrs = attrs,
 };