MPE: Change MPESynthesizer::voicesLock member from private to protected so that sub-classes can lock the voicedLock

modules/juce_audio_basics/mpe/juce_MPESynthesiser.h

@@ -297,11 +297,11 @@ protected:
 
     //==============================================================================
     OwnedArray<MPESynthesiserVoice> voices;
+    CriticalSection voicesLock;
 
 private:
     //==============================================================================
     bool shouldStealVoices;
-    CriticalSection voicesLock;
 
     JUCE_DECLARE_NON_COPYABLE_WITH_LEAK_DETECTOR (MPESynthesiser)
 };