Fix missing includes, close ticket #8457.

e.g. Now can compiler following simple case.

    #include <boost/fusion/iterator/iterator_facade.hpp>
    #include <boost/fusion/iterator/iterator_adapter.hpp>

    struct null_iterator
        : iterator_facade<null_iterator, forward_traversal_tag> {};

    struct null_adapter
        : iterator_adapter<null_adapter, null_iterator> {};
Fix to meet constexpr requirements