Fix memory leak in SampleGrabberCallback

Add virtual destructor to SampleGrabberCallback to let it properly
destruct its subclass when calling 'detete this'

Change-Id: I5f74f82314e22225a0a153c7f4bca531a79dd8f9
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>
Reviewed-by: Yoann Lopes <yoann.lopes@digia.com>

src/plugins/wmf/samplegrabber.h

@@ -72,6 +72,9 @@ public:
 protected:
     SampleGrabberCallback() : m_cRef(1) {}
 
+public:
+    virtual ~SampleGrabberCallback() {}
+
 private:
     long m_cRef;
 };