fix cache limiter error

The barcode label output screen throws this error for me unless the session is initialized before anything else:

Warning: session_start(): Cannot send session cache limiter - headers already sent (output started at /home/plnlabels/labels.palni.org/unm/WMS-Labeling/forms/barcode_label_output.php:7) in /home/plnlabels/labels.palni.org/unm/WMS-Labeling/forms/barcode_label_output.php on line 15