QMetaType: Specialize IsPointerToTypeDerivedFromQObject for const void*

'const void' is not derived from QObject, and is not a type we can use
sizeof to determine that.

Change-Id: I008f444bbed9fc8f9147da0f7f336757f660e224
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@digia.com>
Reviewed-by: Olivier Goffart <ogoffart@woboq.com>

src/corelib/kernel/qmetatype.h

@@ -1277,6 +1277,11 @@ namespace QtPrivate
         enum { Value = false };
     };
     template<>
+    struct IsPointerToTypeDerivedFromQObject<const void*>
+    {
+        enum { Value = false };
+    };
+    template<>
     struct IsPointerToTypeDerivedFromQObject<QObject*>
     {
         enum { Value = true };