Standalone Plug-In: Avoided build errors when including juce_StandeloneFilterWindow.h in a custom standalone plug-in app

modules/juce_audio_plugin_client/Standalone/juce_StandaloneFilterWindow.h

@@ -24,6 +24,10 @@
   ==============================================================================
 */
 
+#if JUCE_MODULE_AVAILABLE_juce_audio_plugin_client
+ extern AudioProcessor* JUCE_API JUCE_CALLTYPE createPluginFilterOfType (AudioProcessor::WrapperType type);
+#endif
+
 namespace juce
 {