Fix a spurious gcc warning

include/boost/beast/core/impl/buffers_cat.ipp

@@ -34,6 +34,8 @@ class buffers_cat_view<Bn...>::const_iterator
 
     struct past_end
     {
+        char unused = 0; // make g++8 happy
+
         operator bool() const noexcept
         {
             return true;