Add back iscsi_queue_pdu

We need a public symbol for iscsi_queue_pdu. This is now just a
simple wrapper around iscsi->t->queue_pdu

https://github.com/sahlberg/libiscsi/issues/212

Signed-off-by: Ronnie Sahlberg <ronniesahlberg@gmail.com>

lib/task_mgmt.c

@@ -81,7 +81,7 @@ iscsi_task_mgmt_async(struct iscsi_context *iscsi,
 	pdu->callback     = cb;
 	pdu->private_data = private_data;
 
-	if (iscsi->t->queue_pdu(iscsi, pdu) != 0) {
+	if (iscsi_queue_pdu(iscsi, pdu) != 0) {
 		iscsi_set_error(iscsi, "failed to queue iscsi taskmgmt pdu");
 		iscsi->t->free_pdu(iscsi, pdu);
 		return -1;