packet: Fix build with INET disabled.

af_packet.c:(.text+0x3d130): undefined reference to `ip_defrag'
or
ERROR: "ip_defrag" [net/packet/af_packet.ko] undefined!

Reported-by: Randy Dunlap <randy.dunlap@oracle.com>
Signed-off-by: David S. Miller <davem@davemloft.net>