netfilter: arp_tables: restore a SPDX identifier

This was accidentally removed in an unrelated commit.

Fixes: c2f12630 ("netfilter: switch nf_setsockopt to sockptr_t")
Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: David S. Miller <davem@davemloft.net>

net/ipv4/netfilter/arp_tables.c

-
+// SPDX-License-Identifier: GPL-2.0-only
 /*
  * Packet matching code for ARP packets.
  *