Fix Backup script mysql

==== Required information ====
- iRedMail version: 0.8.5
- Store mail accounts in which backend (LDAP/MySQL/PGSQL): LDAP
- Linux/BSD distribution name and version: CentOS 6.4
- Related log if you're reporting an issue:
====

Hello
The script (/var/vmail/backup/backup_mysql.sh) contains the following line:

export DATABASES=" mysql mysql policyd amavisd roundcubemail iredadmin"

The that you can make is remove one mysql da linha.

export DATABASES=" mysql policyd amavisd roundcubemail iredadmin"

If you have other databases, include on line to the backup make