Removed SDL_bool in favor of plain bool

We require stdbool.h in the build environment, so we might as well use the plain bool type.

If your environment doesn't have stdbool.h, this simple replacement will suffice:
typedef signed char bool;

test/testaudiohotplug.c

@@ -70,7 +70,7 @@ static void iteration(void)
             }
         } else if (e.type == SDL_EVENT_AUDIO_DEVICE_ADDED) {
             const SDL_AudioDeviceID which = (SDL_AudioDeviceID) e.adevice.which;
-            const SDL_bool recording = e.adevice.recording ? SDL_TRUE : SDL_FALSE;
+            const bool recording = e.adevice.recording ? true : false;
             const char *name = SDL_GetAudioDeviceName(which);
             if (name) {
                 SDL_Log("New %s audio device at id %u: %s", devtypestr(recording), (unsigned int)which, name);