Fixed build problem with SDL_string.c

Officially deprecated SDL_byteorder.h, SDL_getenv.h and SDL_types.h
Moved endian-related SDL_rwops code into SDL_rwops.c

src/stdlib/SDL_qsort.c

@@ -47,8 +47,7 @@
 #include <stdlib.h>
 #include <string.h>
 */
-#include "SDL_stdlib.h"
-#include "SDL_string.h"
+#include "SDL_stdinc.h"
 
 #define assert(X)
 #define malloc	SDL_malloc