Readd accidentally removed forward declaration

This got removed in 74dc8225d1 when I
wanted to rework all `*Impl` classes the same way I did with
`YaPluginFactoryImpl`. This ended up not being possible, but
accidentally also removed this forward declaration. With unity builds
this did not cause issues however, but with regular builds it might
depending on which files are changed.

src/plugin/bridges/vst3.h

@@ -23,7 +23,9 @@
 #include "../../common/logging/vst3.h"
 #include "common.h"
 #include "vst3-impls/plugin-factory.h"
-#include "vst3-impls/plugin-proxy.h"
+
+// Forward declarations
+class Vst3PluginProxyImpl;
 
 /**
  * This handles the communication between the native host and a VST3 plugin