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>