tools/libbpf: signedness bug in btf_dedup_ref_type()

The "ref_type_id" variable needs to be signed for the error handling
to work.

Fixes: d5caef5b ("btf: add BTF types deduplication algorithm")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Andrii Nakryiko <andriin@fb.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>

tools/lib/bpf/btf.c

@@ -2326,7 +2326,8 @@ static int btf_dedup_ref_type(struct btf_dedup *d, __u32 type_id)
 	struct btf_type *t, *cand;
 	/* if we don't find equivalent type, then we are representative type */
 	__u32 new_id = type_id;
-	__u32 h, ref_type_id;
+	int ref_type_id;
+	__u32 h;
 
 	if (d->map[type_id] == BTF_IN_PROGRESS_ID)
 		return -ELOOP;