Make QMetaSequence's d_ptr const

The container interface should really never change.

Change-Id: I31dedf5b776da97a747f0eb26f3bc83ce46f3caa
Reviewed-by: Lars Knoll <lars.knoll@qt.io>

src/corelib/kernel/qmetacontainer.h

@@ -672,7 +672,7 @@ public:
     }
 
 private:
-    QtMetaContainerPrivate::QMetaSequenceInterface *d_ptr = nullptr;
+    const QtMetaContainerPrivate::QMetaSequenceInterface *d_ptr = nullptr;
 };
 
 QT_END_NAMESPACE