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>