Fixed compiler warning

src/thread/SDL_thread.c

@@ -56,6 +56,7 @@ SDL_ThreadsInit(void)
    clean up threads here.  If any threads are still running after this call,
    they will no longer have access to any per-thread data.
  */
+#if 0
 static void
 SDL_ThreadsQuit(void)
 {
@@ -67,6 +68,7 @@ SDL_ThreadsQuit(void)
         SDL_DestroyMutex(mutex);
     }
 }
+#endif
 
 /* Routines for manipulating the thread list */
 static void