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.
OnLinkPrefixLength is not defined on old windows. This is why I made such a PR.

`if (is_vista_or_greater) {` is runtime check, not compile time. Compiling `unicast_address->OnLinkPrefixLength;` in mingw on windows xp will caused compile error.