ip-packet: Fix compiler warning seen on FreeBSD 15

We explicitly use untoh16() so this warning isn't relevant:

  ip_packet.c:313:42: error: taking address of packed member 'ip_len' of class or structure 'ip' may result in an unaligned pointer value [-Werror,-Waddress-of-packed-member]

src/libipsec/ip_packet.c

diff --git a/src/libipsec/ip_packet.c b/src/libipsec/ip_packet.c
index b623c9b..08a4211 100644
--- a/src/libipsec/ip_packet.c
+++ b/src/libipsec/ip_packet.c
@@ -310,7 +310,7 @@
 			}
 			ip = (struct ip*)packet.ptr;
 			/* remove any RFC 4303 TFC extra padding */
-			packet.len = min(packet.len, untoh16(&ip->ip_len));
+			packet.len = min(packet.len, untoh16((void*)&ip->ip_len));
 			payload = chunk_skip(packet, ip->ip_hl * 4);
 			if ((ip->ip_off & htons(IP_OFFMASK)) == 0 &&
 				!parse_transport_header(payload, ip->ip_p, &sport, &dport))