netfilter: ipt_addrtype: rename to xt_addrtype

Followup patch will add ipv6 support.

ipt_addrtype.h is retained for compatibility reasons, but no longer used
by the kernel.

Signed-off-by: Florian Westphal <fwestphal@astaro.com>
Signed-off-by: Patrick McHardy <kaber@trash.net>