samples/bpf: Add missing option to xdp_sample_pkts usage

xdp_sample_pkts usage() is missing the introduction of the
"-S" option, this patch adds it.

Fixes: d50ecc46 ("samples/bpf: Attach XDP programs in driver mode by default")
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Link: https://lore.kernel.org/bpf/20210615135724.29528-1-wanghai38@huawei.com