nuttx/mqueue: fix build break if disable posix message queue

fix build break if enable CONFIG_DISABLE_MQUEUE:

In file included from sched/mqueue/mqueue.h:38,
                 from group/group_leave.c:45:
include/nuttx/mqueue.h:124:26: error: ‘CONFIG_FS_MQUEUE_NPOLLWAITERS’ undeclared here (not in a function); did you mean ‘CONFIG_SERIAL_NPOLLWAITERS’?
  124 |   FAR struct pollfd *fds[CONFIG_FS_MQUEUE_NPOLLWAITERS];
      |                          ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~
      |                          CONFIG_SERIAL_NPOLLWAITERS

Signed-off-by: chao an <anchao@lixiang.com>

include/nuttx/mqueue.h

@@ -78,6 +78,10 @@
 #  define _MQ_TIMEDRECEIVE(d,m,l,p,t) mq_timedreceive(d,m,l,p,t)
 #endif
 
+#ifndef CONFIG_FS_MQUEUE_NPOLLWAITERS
+#  define CONFIG_FS_MQUEUE_NPOLLWAITERS 0
+#endif
+
 #if CONFIG_FS_MQUEUE_NPOLLWAITERS > 0
 #  define nxmq_pollnotify(msgq, eventset) \
    poll_notify(msgq->fds, CONFIG_FS_MQUEUE_NPOLLWAITERS, eventset)