Fix missing includes, close ticket #8457.

e.g. Now can compile 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> {};

include/boost/fusion/iterator/iterator_facade.hpp

@@ -14,6 +14,7 @@
 #include <boost/fusion/support/category_of.hpp>
 #include <boost/type_traits/is_same.hpp>
 #include <boost/mpl/assert.hpp>
+#include <boost/mpl/if.hpp>
 
 namespace boost { namespace fusion
 {