Add "recordings" to the backup exclude list

Add "recordings" to the backup exclude list

Since https://github.com/tvheadend/tvheadend/pull/1540, enabled by
https://github.com/tvheadend/tvheadend/pull/1535 and
https://github.com/tvheadend/tvheadend/pull/1538, we have been
storing the recordings in a subdirectory of the configuration
directory by default.  Because of this, the recordings are getting
stored in the configuration backup.  This causes the backups to
take forever and fill the disk (see
https://github.com/tvheadend/tvheadend/issues/1625).  Instead,
exclude the "recordings" directory from the backup to prevent this.