libbpf: BTF is required when externs are present

BTF is required to get type information about extern variables.
Signed-off-by: NAndrii Nakryiko <andriin@fb.com>
Signed-off-by: NAlexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20191219002837.3074619-4-andriin@fb.com

tools/lib/bpf/libbpf.c

@@ -1887,7 +1887,8 @@ static void bpf_object__sanitize_btf_ext(struct bpf_object *obj)
 
 static bool bpf_object__is_btf_mandatory(const struct bpf_object *obj)
 {
-	return obj->efile.btf_maps_shndx >= 0;
+	return obj->efile.btf_maps_shndx >= 0 ||
+	       obj->nr_extern > 0;
 }
 
 static int bpf_object__init_btf(struct bpf_object *obj,