group: suppress warnings for unused variables in case of empty types

src/entt/entity/group.hpp

@@ -28,7 +28,7 @@ class extended_group_iterator;
 template<typename It, typename... Owned, typename... Get>
 class extended_group_iterator<It, owned_t<Owned...>, get_t<Get...>> {
     template<typename Type>
-    auto index_to_element(Type &cpool) const {
+    auto index_to_element([[maybe_unused]] Type &cpool) const {
         if constexpr(ignore_as_empty_v<typename Type::value_type>) {
             return std::make_tuple();
         } else {