libbpf: fix error code returned on corrupted ELF

All of libbpf errors are negative, except this one. Fix it.
Acked-by: NSong Liu <songliubraving@fb.com>
Signed-off-by: NAndrii Nakryiko <andriin@fb.com>
Signed-off-by: NDaniel Borkmann <daniel@iogearbox.net>

tools/lib/bpf/libbpf.c

@@ -1221,7 +1221,7 @@ static int bpf_object__elf_collect(struct bpf_object *obj, int flags)
 
 	if (!obj->efile.strtabidx || obj->efile.strtabidx >= idx) {
 		pr_warning("Corrupted ELF file: index of strtab invalid\n");
-		return LIBBPF_ERRNO__FORMAT;
+		return -LIBBPF_ERRNO__FORMAT;
 	}
 	if (btf_data) {
 		obj->btf = btf__new(btf_data->d_buf, btf_data->d_size);