add missing include

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@9839 a83610d8-ad2a-0410-a6ab-fc0612d85776
fix msvc-7.1 build

git-svn-id: https://svn.code.sf.net/p/libtorrent/code/trunk@9840 a83610d8-ad2a-0410-a6ab-fc0612d85776