Fix ODR-used violations.

include/boost/fusion/iterator/deref_data.hpp

@@ -41,7 +41,7 @@ namespace boost { namespace fusion
 
     template <typename It>
     BOOST_CONSTEXPR BOOST_FUSION_GPU_ENABLED
-    typename result_of::deref_data<It>::type
+    inline typename result_of::deref_data<It>::type
     deref_data(It const& it)
     {
         return result_of::deref_data<It>::call(it);