Extracted enable_if_convertible trait to a separate header.

Keep including the new header in iterator_adaptor.hpp for backward
compatibility, until downstream users are updated to include the new
header.

include/boost/iterator/reverse_iterator.hpp

@@ -8,6 +8,7 @@
 #define BOOST_REVERSE_ITERATOR_23022003THW_HPP
 
 #include <boost/iterator/iterator_adaptor.hpp>
+#include <boost/iterator/enable_if_convertible.hpp>
 
 namespace boost {
 namespace iterators {