sysmocom-backup-data: Fix the glob to load additional files list

The defaults.files ends with files and not backup.

recipes-apps/sysmocom-backup-data/files/sysmocom-backup-data

@@ -7,7 +7,7 @@ FILES="etc/sysmocom/backup.d"
 
 # Pick some extra files
 if [ -e /etc/sysmocom/backup.d/ ]; then
-	for extra in `ls /etc/sysmocom/backup.d/*.backup`;
+	for extra in `ls /etc/sysmocom/backup.d/*.files`;
 	do
 		echo "Add extras from $extra."
 		FILES="$FILES `cat $extra`"