fix warning when building without IPv6 support

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@8664 a83610d8-ad2a-0410-a6ab-fc0612d85776
fix windows build

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@8665 a83610d8-ad2a-0410-a6ab-fc0612d85776