win: fix lost of OnLinkPrefixLength on xp.

Use OnLinkPrefixLength will cause compile error on windows xp.
sizeof(IP_ADAPTER_UNICAST_ADDRESS_XP) = 48, is aligned.
Well, that isn't exactly the most portable way of doing it. Can you do something like:

`` prefix_len = ((_IP_ADAPTER_UNICAST_ADDRESS_LH *) unicast_address)->OnLinkPrefixLength;
``