net: if_arp: add ARPHRD_NETLINK type

This small patch adds the definition of ARPHRD_NETLINK which can for
example be used by netlink monitoring devices as device type. So that
sockaddr_ll can pick it up and based on that choose the correct packet
dissector.

Signed-off-by: Daniel Borkmann <dborkman@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>