Fix attr name in prune script

scripts/prune_packets.py

@@ -22,8 +22,8 @@ def pack_hdr_bytes(hdr):
 with open(args.outfile, 'wb') as fptr:
     idx = 0
     for pkt in stream.PacketStream(open(args.pdsfile)):
-        h1 = pkt._primary_header
-        h2 = pkt._secondary_header
+        h1 = pkt.primary_header
+        h2 = pkt.secondary_header
         if h2:
             h1.data_length_minus1 = c.sizeof(h2) - 1
             fptr.write(h1)