[arch-general] An empty /etc/crontab file maybe needed bo be add in to package filesystem.

[Techlive Zheng]

Due to cronie doesn't contain an /etc/crontab anymore, there are losts
of msgs of missing crontab as shown below in errors.log.

> /usr/sbin/crond[1236]: (root) CAN'T OPEN (/etc/crontab): No such file or 
> directory