Change the return type of AccessValues::size() to be view-like

This also fixes a recently introduced warning.

Author: aprokop

src/spatial/detail/ArborX_AccessTraits.hpp

@@ -198,7 +198,10 @@ struct AccessTraits<Details::AccessValuesI<Values>>
   }
 
   KOKKOS_FUNCTION
-  static decltype(auto) size(AccessValues const &w) { return w.size(); }
+  static typename memory_space::size_type size(AccessValues const &w)
+  {
+    return w.size();
+  }
 };
 
 } // namespace ArborX