[include/rtthread.h] Add function declarations to avoid compilation warnings.

include/rtthread.h

@@ -410,6 +410,7 @@ rt_err_t rt_mb_send(rt_mailbox_t mb, rt_ubase_t value);
 rt_err_t rt_mb_send_wait(rt_mailbox_t mb,
                          rt_ubase_t  value,
                          rt_int32_t   timeout);
+rt_err_t rt_mb_urgent(rt_mailbox_t mb, rt_ubase_t value);
 rt_err_t rt_mb_recv(rt_mailbox_t mb, rt_ubase_t *value, rt_int32_t timeout);
 rt_err_t rt_mb_control(rt_mailbox_t mb, int cmd, void *arg);
 #endif