QProperty: make a few helpers constexpr

Change-Id: I1f3b2223530c311a7b40fd36c8162e32adbd9569
Reviewed-by: Ulf Hermann <ulf.hermann@qt.io>

src/corelib/kernel/qproperty.h

@@ -228,7 +228,7 @@ public:
                            // placeholder. We only can store 4 different values, therefore those two conflate
     };
 
-    QPropertyObserver() = default;
+    constexpr QPropertyObserver() = default;
     QPropertyObserver(QPropertyObserver &&other);
     QPropertyObserver &operator=(QPropertyObserver &&other);
     ~QPropertyObserver();

src/corelib/kernel/qpropertyprivate.h

@@ -126,7 +126,7 @@ template <typename T, typename Tag>
 class QTagPreservingPointerToPointer
 {
 public:
-    QTagPreservingPointerToPointer() = default;
+    constexpr QTagPreservingPointerToPointer() = default;
 
     QTagPreservingPointerToPointer(T **ptr)
         : d(reinterpret_cast<quintptr*>(ptr))