netfilter: arp_tables: restore a SPDX identifier

This was accidentally removed in an unrelated commit.

Fixes: c2f12630c60f ("netfilter: switch nf_setsockopt to sockptr_t")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David S. Miller <davem@davemloft.net>
1 file changed