Use the #3669 fixture's optional member in to_json

Issue3669Holder::d is never read, so clang's -Weverything -Werror build
(ci_test_clang) fails with -Wunused-private-field. Reference the member
in the hidden-friend to_json; this does not affect the instantiation
cycle the fixture reproduces.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>

tests/src/unit-regression2.cpp

@@ -280,8 +280,9 @@ class Issue3669Holder
     // GCC < 11 (C++11/14) rejects a free to_json(json&, const Issue3669Holder&)
     // here, because ADL for Issue3669Dummy finds it and closes an instantiation
     // cycle; a hidden friend is only visible to ADL for Issue3669Holder
-    friend void to_json(json& j, const Issue3669Holder& /*unused*/)
+    friend void to_json(json& j, const Issue3669Holder& h)
     {
+        static_cast<void>(h.d); // silence -Wunused-private-field
         j = "holder";
     }
 };