added "explicit" to iterator_adaptor constructor

[SVN r9234]

include/boost/iterator_adaptors.hpp

@@ -509,6 +509,7 @@ struct iterator_adaptor :
  public:
     iterator_adaptor() { }
 
+    explicit
     iterator_adaptor(const Base& it, const Policies& p = Policies())
         : m_iter_p(it, p) {
       policies().initialize(iter());