btf__add_btf
ret = btf__add_btf(combined, mod);
LIBBPF_API int btf__add_btf(struct btf *btf, const struct btf *src_btf);
id = btf__add_btf(btf1, btf2);
id = btf__add_btf(combined, split1);
id = btf__add_btf(combined, split2);