fix(command_backup): Correct path for exludedir variable

The current implementation doesn't use the full file path to determine the existence of the backup directory in relation to the server which means the current stats just looks for the "lgsm/backup" directory. That may work if you're already in the root directory for the server but if you're scripting the backups via cron, you're not likely going to be invoking the backup from the root directory.

lgsm/functions/command_backup.sh

@@ -132,7 +132,7 @@ fn_backup_compression(){
 	excludedir=$(fn_backup_relpath)
 
 	# Check that excludedir is a valid path.
-	if [ ! -d "${excludedir}" ] ; then
+	if [ ! -d "${rootdir}/${excludedir}" ] ; then
 		fn_print_fail_nl "Problem identifying the previous backup directory for exclusion."
 		fn_script_log_fatal "Problem identifying the previous backup directory for exclusion"
 		core_exit.sh