net-p2p/bitcoin: unbreak with boost 1.72

wallet/walletutil.cpp:77:23: error: no member named 'level' in 'boost::filesystem::recursive_directory_iterator'

} else if (it.level() == 0 && it->symlink_status().type() == fs::regular_file && IsBerkeleyBtree(it...
           ~~ ^

Reported by: antoine (via bug 241449 exp-run)