audio: Cleaned out most remaining /* */ comments for // style.

Fully committing to it...!

This left SDL_wave.* alone for now, since there's a ton of comments in there
and this code hasn't changed much from SDL2 so far. But as SDL2 ages out a
little more, I'll likely switch this over, too.

src/audio/emscripten/SDL_emscriptenaudio.h

@@ -30,4 +30,4 @@ struct SDL_PrivateAudioData
     Uint8 *mixbuf;
 };
 
-#endif /* SDL_emscriptenaudio_h_ */
+#endif // SDL_emscriptenaudio_h_