Replace { } with BOOST_DEFAULTED_FUNCTION for C++03 compatibility.

Author: jaykrell

include/boost/iostreams/filtering_stream.hpp

@@ -93,7 +93,9 @@ class filtering_stream_base
                  typename Chain::char_type, 
                  typename Chain::traits_type
             >::stream_type                                stream_type;
-    ~filtering_stream_base() BOOST_NOEXCEPT = default;
+
+    BOOST_DEFAULTED_FUNCTION(~filtering_stream_base() BOOST_NOEXCEPT, { })
+
     filtering_stream_base() : stream_type(0) { this->set_chain(&chain_); }
 private:
     void notify() { this->rdbuf(chain_.empty() ? 0 : &chain_.front()); }