Fixed warning: no previous prototype for function 'SDL_UpdateAudio'

src/audio/SDL_audio.c

@@ -20,6 +20,7 @@
 */
 #include "SDL_internal.h"
 
+#include "SDL_audio_c.h"
 #include "SDL_sysaudio.h"
 #include "../thread/SDL_systhread.h"
 #include "../SDL_utils_c.h"