Apply suggestion by @iwanders under PR #5334

Author: rwgk

include/pybind11/detail/struct_smart_holder.h

@@ -234,7 +234,7 @@ struct smart_holder {
     // Caller is responsible for precondition: ensure_compatible_rtti_uqp_del<T, D>() must succeed.
     template <typename T, typename D>
     std::unique_ptr<D> extract_deleter(const char *context) const {
-        auto *gd = std::get_deleter<guarded_delete>(vptr);
+        const auto *gd = std::get_deleter<guarded_delete>(vptr);
         if (gd && gd->use_del_fun) {
             const auto &custom_deleter_ptr = gd->del_fun.template target<custom_deleter<T, D>>();
             if (custom_deleter_ptr == nullptr) {