Add explicit destructors to the unordered containers. Refs #2908.

Isn't really needed but it doesn't hurt.

[SVN r53253]

include/boost/unordered/unordered_set.hpp

@@ -105,6 +105,8 @@ namespace boost
             : base(f, l, n, hf, eql, a)
         {
         }
+        
+        ~unordered_set() {}
 
 #if defined(BOOST_HAS_RVALUE_REFS)
         unordered_set(unordered_set&& other)
@@ -528,6 +530,8 @@ namespace boost
         {
         }
 
+        ~unordered_multiset() {}
+
 #if defined(BOOST_HAS_RVALUE_REFS)
         unordered_multiset(unordered_multiset&& other)
             : base(other.base, boost::unordered_detail::move_tag())