Fix ODR-used violations.

include/boost/fusion/iterator/next.hpp

@@ -55,7 +55,7 @@ namespace boost { namespace fusion
 
     template <typename Iterator>
     BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
-    typename result_of::next<Iterator>::type const
+    inline typename result_of::next<Iterator>::type const
     next(Iterator const& i)
     {
         return result_of::next<Iterator>::call(i);