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;

src/stdlib/SDL_malloc.c

@@ -6390,7 +6390,7 @@ void SDL_GetMemoryFunctions(SDL_malloc_func *malloc_func,
     }
 }
 
-SDL_bool SDL_SetMemoryFunctions(SDL_malloc_func malloc_func,
+bool SDL_SetMemoryFunctions(SDL_malloc_func malloc_func,
                                 SDL_calloc_func calloc_func,
                                 SDL_realloc_func realloc_func,
                                 SDL_free_func free_func)