poly: mark functions [[nodiscard]] as needed

src/entt/poly/poly.hpp

@@ -30,11 +30,11 @@ struct poly_inspector {
      * @return A poly inspector convertible to any type.
      */
     template<std::size_t Member, typename... Args>
-    poly_inspector invoke(Args &&...args) const;
+    [[nodiscard]] poly_inspector invoke(Args &&...args) const;
 
     /*! @copydoc invoke */
     template<std::size_t Member, typename... Args>
-    poly_inspector invoke(Args &&...args);
+    [[nodiscard]] poly_inspector invoke(Args &&...args);
 };
 
 /**