Forward-declare PaUtilRingBuffer

This is not strictly necessary but it helped the static analyzer in my
editor ...

Author: mgeier

src/rtmixer.h

@@ -5,6 +5,7 @@ typedef unsigned long PaStreamCallbackFlags;
 /* End of declarations from portaudio.h */
 
 typedef unsigned long frame_t;
+typedef struct PaUtilRingBuffer PaUtilRingBuffer;
 typedef _Bool bool;
 #define true 1
 #define false 0