QSharedData: delete the copy assignment operator

Do not merely declare it as private, use C++11's = delete.
This has the nice side effect that subclasses are no longer
implicitly copy assignable either (they shouldn't be).

Change-Id: Icd03f71006c31baf7d079365fa3bea1a2a9d559b
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>

src/corelib/tools/qshareddata.h

@@ -60,9 +60,8 @@ public:
     inline QSharedData() : ref(0) { }
     inline QSharedData(const QSharedData &) : ref(0) { }
 
-private:
     // using the assignment operator would lead to corruption in the ref-counting
-    QSharedData &operator=(const QSharedData &);
+    QSharedData &operator=(const QSharedData &) = delete;
 };
 
 template <class T> class QSharedDataPointer