Use use_default from Boost.Core

boost::use_default is now defined in Core for multiple Boost libraries.

include/boost/iterator/detail/facade_iterator_category.hpp

@@ -4,6 +4,8 @@
 #ifndef FACADE_ITERATOR_CATEGORY_DWA20031118_HPP
 # define FACADE_ITERATOR_CATEGORY_DWA20031118_HPP
 
+# include <boost/core/use_default.hpp>
+
 # include <boost/iterator/iterator_categories.hpp>
 
 # include <boost/mpl/or.hpp>  // used in iterator_tag inheritance logic
@@ -33,8 +35,7 @@
 namespace boost {
 namespace iterators {
 
-// forward declaration
-struct use_default;
+using boost::use_default;
 
 namespace detail {