tuple: silence gcc warning

fix warning:

```
../../../boost/tuple/detail/tuple_basic.hpp: In function ‘typename boost::tuples::access_traits<typename boost::tuples::element<N, boost::tuples::cons<HT, TT> >::type>::const_type boost::tuples::get(const boost::tuples::cons<HT, TT>&)’:
../../../boost/tuple/detail/tuple_basic.hpp:228:45: warning: typedef ‘cons_element’ locally defined but not used [-Wunused-local-typedefs]
   typedef BOOST_DEDUCED_TYPENAME impl::type cons_element;
```