btf__add_restrict
LIBBPF_API int btf__add_restrict(struct btf *btf, int ref_type_id);
btf__add_restrict(btf2, 7); /* [21] restrict union (anon) */
btf__add_restrict(btf, 3); /* [12] restrict int *; */
id = btf__add_restrict(btf, 4);