Don't export SDL_AudioInit/Quit, use SDL_InitSubsystem instead (#6912)

* Don't export SDL_AudioInit/Quit, use SDL_InitSubsystem instead

* Update README

Co-authored-by: Sam Lantinga <slouken@libsdl.org>

src/SDL.c

@@ -37,6 +37,7 @@
 
 #include "SDL_assert_c.h"
 #include "SDL_log_c.h"
+#include "audio/SDL_audio_c.h"
 #include "events/SDL_events_c.h"
 #include "haptic/SDL_haptic_c.h"
 #include "joystick/SDL_joystick_c.h"