tst_qflatmap: fix -Wunused-local-typedef (Clang)

False Positive, of course, since it's used in dependent callee
transparent_impl().

Amends 58e8ae5605.

Pick-to: 6.3
Change-Id: I7520fb7cf6bce28a5fd9381783ba349119309132
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>

tests/auto/corelib/tools/qflatmap/tst_qflatmap.cpp

@@ -511,7 +511,7 @@ void tst_QFlatMap::transparency_using()
 {
     struct StringViewCompare
     {
-        using is_transparent = void;
+        using is_transparent [[maybe_unused]] = void;
         bool operator()(QAnyStringView lhs, QAnyStringView rhs) const
         {
             return lhs < rhs;