Ensure each container type can be explicitly instantiated

include/boost/unordered/concurrent_node_set.hpp

@@ -194,6 +194,7 @@ namespace boost {
       {
       }
 
+      template <bool avoid_explicit_instantiation = true>
       concurrent_node_set(
         unordered_node_set<Key, Hash, Pred, Allocator>&& other)
           : table_(std::move(other.table_))