Revert "Replace header guards with #pragma once"

This reverts commit a6bc896e24.

src/AudioIOBufferHelper.h

@@ -1,4 +1,5 @@
-#pragma once
+#ifndef AUDIOIO_BUFFER_HELPER_H
+#define AUDIOIO_BUFFER_HELPER_H
 
 #include "AudioIO.h"
 #include "../libraries/lib-utility/MemoryX.h"
@@ -37,3 +38,5 @@ class AudioIOBufferHelper
             delete[] chans;
         }
 };
+
+#endif