Resolve symbol ambiguity when building with Unity Build

In unity build, we may get `error: reference to 'detail' is ambiguous`
between the `detail` namespace defined in `qpropertyprivate.h`, and
`qflatmap_p.h`. For now, this causes an issue during the compilation of
`qcalendar.cpp` and it may occur in other places where qflatmap is
included.

Pick-to: 6.5
Task-number: QTBUG-109394
Change-Id: Ie4bbb66543d26a5db58488e924333d98ce0adebf
Reviewed-by: Joerg Bornemann <[email protected]>
Reviewed-by: Fabian Kosmale <[email protected]>
Reviewed-by: Qt CI Bot <[email protected]>

src/corelib/tools/qflatmap_p.h

@@ -83,6 +83,7 @@ class QFlatMapValueCompare : protected Compare
     }
 };
 
+namespace qflatmap {
 namespace detail {
 template <class T>
 class QFlatMapMockPointer
@@ -100,15 +101,17 @@ class QFlatMapMockPointer
     }
 };
 } // namespace detail
+} // namespace qflatmap
 
 template<class Key, class T, class Compare = std::less<Key>, class KeyContainer = QList<Key>,
          class MappedContainer = QList<T>>
 class QFlatMap : private QFlatMapValueCompare<Key, T, Compare>
 {
     static_assert(std::is_nothrow_destructible_v<T>, "Types with throwing destructors are not supported in Qt containers.");
 
-    template <class U>
-    using mock_pointer = detail::QFlatMapMockPointer<U>;
+    template<class U>
+    using mock_pointer = qflatmap::detail::QFlatMapMockPointer<U>;
+
 public:
     using key_type = Key;
     using mapped_type = T;