[pthreads]Add component init for pthread_system_init.

components/libc/pthreads/pthread.c

@@ -39,6 +39,7 @@ int pthread_system_init(void)
 
     return 0;
 }
+INIT_COMPONENT_EXPORT(pthread_system_init);
 
 static void _pthread_cleanup(rt_thread_t tid)
 {