drivers/rptun: guard nxsched_waitpid with CONFIG_SCHED_WAITPID

nxsched_waitpid is only available when CONFIG_SCHED_WAITPID is
enabled. Wrap the call with #ifdef to fix the build error when
this option is disabled.

Signed-off-by: huojianchao <huojianchao@xiaomi.com>
Signed-off-by: Bowen Wang <wangbowen6@xiaomi.com>

drivers/rptun/rptun.c

@@ -932,7 +932,9 @@ static int rptun_dev_stop(FAR struct remoteproc *rproc)
     {
       priv->stop = true;
       nxsig_kill(priv->pid, SIGKILL);
+#ifdef CONFIG_SCHED_WAITPID
       nxsched_waitpid(priv->pid, NULL, WEXITED);
+#endif
       priv->stop = false;
       priv->pid = -EINVAL;
     }