Include SDL.h in sd-sdl/sound.c to get SDL_INIT_AUDIO define

I tried compiling PUAE master on Mac OS X (10.9) and ran into some compiler errors:

```
sound.c:96:6: warning: implicit declaration of function 'SDL_InitSubSystem' is invalid in C99 [-Wimplicit-function-declaration]
        if (SDL_InitSubSystem (SDL_INIT_AUDIO) == 0) {
            ^
sound.c:96:25: error: use of undeclared identifier 'SDL_INIT_AUDIO'
        if (SDL_InitSubSystem (SDL_INIT_AUDIO) == 0) {
                               ^
sound.c:107:7: warning: implicit declaration of function 'SDL_QuitSubSystem' is invalid in C99 [-Wimplicit-function-declaration]
                    SDL_QuitSubSystem (SDL_INIT_AUDIO);
                    ^
sound.c:107:26: error: use of undeclared identifier 'SDL_INIT_AUDIO'
                    SDL_QuitSubSystem (SDL_INIT_AUDIO);
```

In my copy of the `SDL.framework` for OS X (version 1.2.15) the identifiers `SDL_InitSubSystem`, `SDL_QuitSubSystem`, and `SDL_INIT_AUDIO` is defined in `SDL.h`, so I made this patch and then I was able to compile the project OK. Including `SDL.h` will pull in `SDL_audio.h` as well.