Add virtual destructors to two classes with other virtuals

Classes with virtuals are used polymorphically (why have virtuals
otherwise?), so they need virtual destructors to be deleted
properly. Unless they are never deleted using objects of those two
classes -- but why have the interface in the first place if that's the
case?

This is binary incompatible change. It was agreed upon in the mailing
list.

Change-Id: I697e4bd53251452a0e6b0c09edd08c4835f90cbd
Discussed-on: http://lists.qt-project.org/pipermail/development/2012-December/008908.html
Reviewed-by: Lars Knoll <lars.knoll@digia.com>

src/multimedia/audio/qaudiosystemplugin.cpp

@@ -44,6 +44,10 @@
 
 QT_BEGIN_NAMESPACE
 
+QAudioSystemFactoryInterface::~QAudioSystemFactoryInterface()
+{
+}
+
 /*!
     \class QAudioSystemPlugin
     \brief The QAudioSystemPlugin class provides an abstract base for audio plugins.