Fix to compile with newer Visual C++, such as 16.11, and /Zc:implicit…

…Noexcept-.

Otherwise gets:
  Error C2694 'override': overriding virtual function
  has less restrictive exception specification than base class virtual
  member function 'base'

Similar changes are being made e.g.:
  boostorg/json#636

include/boost/iostreams/filtering_stream.hpp

@@ -93,6 +93,9 @@ class filtering_stream_base
                  typename Chain::char_type, 
                  typename Chain::traits_type
             >::stream_type                                stream_type;
+
+    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()); }