Use static variable for AudioEngineStub instance

Author: adazem009

src/audio/internal/audioenginestub.cpp

@@ -4,11 +4,11 @@
 
 using namespace libscratchcpp;
 
-AudioEngineStub AudioEngineStub::instance;
+static AudioEngineStub INSTANCE;
 
 IAudioEngine *IAudioEngine::instance()
 {
-    return &AudioEngineStub::instance;
+    return &INSTANCE;
 }
 
 AudioEngineStub::AudioEngineStub()

src/audio/internal/audioenginestub.h

@@ -17,7 +17,6 @@ class AudioEngineStub : public IAudioEngine
         void setVolume(float volume) override;
 
     private:
-        static AudioEngineStub instance;
         float m_volume = 1.0f;
 };