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.
_IP_ADAPTER_UNICAST_ADDRESS_LH should be defined in the Windows SDK for all versions of Visual Studio we support.
For mingw, you can defined it in winsock.h like we do for _IP_ADAPTER_UNICAST_ADDRESS_XP: https://github.com/joyent/libuv/blob/master/src/win/winsock.h#L150