Added missing virtual destructor to StandalonePluginHolder.

modules/juce_audio_plugin_client/Standalone/juce_StandaloneFilterWindow.h

@@ -53,7 +53,7 @@ public:
         startPlaying();
     }
 
-    ~StandalonePluginHolder()
+    virtual ~StandalonePluginHolder()
     {
         deletePlugin();
         shutDownAudioDevices();