selinux: constify nf_hook_ops

nf_hook_ops are not supposed to change at runtime. nf_register_net_hooks
and nf_unregister_net_hooks are working with const nf_hook_ops.
So mark the non-const nf_hook_ops structs as const.

Signed-off-by: Arvind Yadav <arvind.yadav.cs@gmail.com>
Signed-off-by: Paul Moore <paul@paul-moore.com>