Fix openh264 compile error on MacOS

src/ringbuffer/ringbuffer.cpp

@@ -0,0 +1,6 @@
+#include "ringbuffer.h"
+
+int RingBufferDummy()
+{
+    return 0;
+}

src/ringbuffer/ringbuffer.h

@@ -3,6 +3,8 @@
 
 #include <string.h>
 
+int RingBufferDummy();
+
 class Data {
  public:
   Data() = default;