VST3 Client: Fix build when JUCE_VST3_CAN_REPLACE_VST2 is enabled

modules/juce_audio_plugin_client/VST3/juce_VST3_Wrapper.cpp

@@ -65,6 +65,7 @@ JUCE_BEGIN_NO_SANITIZE ("vptr")
 
  namespace Vst2
  {
+ struct AEffect;
  #include "pluginterfaces/vst2.x/vstfxstore.h"
  }
 

modules/juce_audio_processors/utilities/juce_ExtensionsVisitor.h

@@ -44,6 +44,21 @@ typedef struct ComponentInstanceRecord* AudioComponentInstance;
 typedef AudioComponentInstance AudioUnit;
 
 //==============================================================================
+/*  If you are including the VST headers inside a namespace this forward
+    declaration may cause a collision with the contents of `aeffect.h`.
+
+    If that is the case you can avoid the collision by placing a `struct AEffect;`
+    forward declaration inside the namespace and before the inclusion of the VST
+    headers, e.g. @code
+
+    namespace Vst2
+    {
+    struct AEffect;
+    #include <pluginterfaces/vst2.x/aeffect.h>
+    #include <pluginterfaces/vst2.x/aeffectx.h>
+    }
+    @endcode
+*/
 struct AEffect;
 
 //==============================================================================