rewrite source to compile against new public APIs of FluidSynth 2

Source/FluidSynthModel.h

@@ -74,7 +74,7 @@ private:
 
     float currentSampleRate;
 
-    const fluid_preset_t getFirstPreset();
+    fluid_preset_t* getFirstPreset();
     void selectFirstPreset();
     unique_ptr<BankAndPreset> getFirstBankAndPreset();
 
@@ -91,4 +91,4 @@ private:
     ListenerList<Listener> eventListeners;
 
     JUCE_DECLARE_NON_COPYABLE_WITH_LEAK_DETECTOR (FluidSynthModel)
-};
+};