Fix unused static variable warning on OS X

- Prevent some static variables from getting declared
  when including vorbis.h since we don't need them and
  fortunately they provide a mechanism to prevent this.

Core/Contents/Source/PolySound.cpp

@@ -21,7 +21,9 @@
 */
 
 #include "PolySound.h"
+#define OV_EXCLUDE_STATIC_CALLBACKS
 #include <vorbis/vorbisfile.h>
+#undef OV_EXCLUDE_STATIC_CALLBACKS
 #include "PolyString.h"
 #include "PolyLogger.h"