transmission: add 'incomplete' and 'watch' dirs to ujail mounts
To fix the errors:
Sun Apr 10 14:19:41 2022 daemon.err transmission-daemon[29831]: [2022-04-10 14:19:41.098] watchdir Failed to open directory "/mnt/sda1/openwrt/transmission/watch" (2): No such file or directory (watchdir.c:358)