tools: fix unpack in example sysvmq audit reader

Fix the unpack in the example sysvmq audit reader script to
correctly unpack the message type, which is an native long.

From the msgrcv perl docoumentation:

  Note that when a message is received, the message type as a native
  long integer will be the first thing in VAR, followed by the actual
  message.  This packing may be opened with "unpack("l! a*")".

Change-Id: I5c5480c30d530b384d8057fb071b01e67f1b4ad2
Reviewed-on: http://gerrit.openafs.org/10445
Reviewed-by: D Brashear <shadow@your-file-system.com>
Tested-by: D Brashear <shadow@your-file-system.com>