net/kea: unbreak with boost 1.68

libdhcp++.cc:420:24: error: call to 'distance' is ambiguous

num_defs = distance(range.first, range.second);
           ^~~~~~~~

libdhcp++.cc:428:24: error: call to 'distance' is ambiguous

num_defs = distance(range.first, range.second);
           ^~~~~~~~

libdhcp++.cc:537:24: error: call to 'distance' is ambiguous

num_defs = distance(range.first, range.second);
           ^~~~~~~~

libdhcp++.cc:545:24: error: call to 'distance' is ambiguous

num_defs = distance(range.first, range.second);
           ^~~~~~~~

libdhcp++.cc:637:31: error: call to 'distance' is ambiguous

size_t num_defs = distance(range.first, range.second);
                  ^~~~~~~~

libdhcp++.cc:743:35: error: call to 'distance' is ambiguous

size_t num_defs = distance(range.first, range.second);
                  ^~~~~~~~

/usr/include/c++/v1/iterator:511:1: note: candidate function [with _InputIter = boost::multi_index::detail::hashed_index_iterator<boost::multi_index::detail::hashed_index_node<boost::multi_index::detail::hashed_index_node<boost::multi_index::detail::index_node_base<boost::shared_ptr<isc::dhcp::OptionDefinition>, std::1::allocator<boost::shared_ptr<isc::dhcp::OptionDefinition> > >, boost::multi_index::detail::hashed_non_unique_tag>, boost::multi_index::detail::hashed_non_unique_tag>, boost::multi_index::detail::bucket_array<std::1::allocator<boost::shared_ptr<isc::dhcp::OptionDefinition> > >, boost::multi_index::detail::hashed_index_global_iterator_tag>]
distance(_InputIter first, _InputIter last)
^
/usr/local/include/boost/iterator/distance.hpp:49:9: note: candidate function [with SinglePassIterator = boost::multi_index::detail::hashed_index_iterator<boost::multi_index::detail::hashed_index_node<boost::multi_index::detail::hashed_index_node<boost::multi_index::detail::index_node_base<boost::shared_ptr<isc::dhcp::OptionDefinition>, std::1::allocator<boost::shared_ptr<isc::dhcp::OptionDefinition> > >, boost::multi_index::detail::hashed_non_unique_tag>, boost::multi_index::detail::hashed_non_unique_tag>, boost::multi_index::detail::bucket_array<std::1::allocator<boost::shared_ptr<isc::dhcp::OptionDefinition> > >, boost::multi_index::detail::hashed_index_global_iterator_tag>]

distance(SinglePassIterator first, SinglePassIterator last)
^

PR: 229569