libretro: Fix compile error with recent sound code changes.

src/common/SoundSDL2.cxx

@@ -58,7 +58,6 @@ SoundSDL2::SoundSDL2(OSystem& osystem, AudioSettings& audioSettings)
 
   queryHardware(myDevices);
 
-
   SDL_zero(myHardwareSpec);
   if(!openDevice())
     return;

src/libretro/SoundLIBRETRO.cxx

@@ -120,4 +120,9 @@ void SoundLIBRETRO::dequeue(Int16* stream, uInt32* samples)
   *samples = outIndex / 2;
 }
 
+// - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
+void SoundLIBRETRO::queryHardware(VariantList& devices)
+{
+}
+
 #endif  // SOUND_SUPPORT

src/libretro/SoundLIBRETRO.hxx

@@ -104,7 +104,6 @@ class SoundLIBRETRO : public Sound
     */
     string about() const override { return ""; }
 
-  public:
     /**
       Empties the playback buffer.
 
@@ -113,6 +112,14 @@ class SoundLIBRETRO : public Sound
     */
     void dequeue(Int16* stream, uInt32* samples);
 
+  protected:
+    /**
+      This method is called to query the audio devices.
+
+      @param devices  List of device names
+    */
+    void queryHardware(VariantList& devices) override;
+
   private:
     // Indicates if the sound device was successfully initialized
     bool myIsInitializedFlag;