In case you might not be aware of this already, let me first point out what might seem to be the obvious but: "there is no such thing as unique packet id in real networks" and since pcap traces are designed to contain dumps of real packets in real networks, there is zero chance you will be able to find a unique packet id in a pcap trace generated by ns-3.
Now, ns-3 does contain a per-packet unique id that is available with the Packet::GetId method and you can trivially change the source code of the function that generates your ascii dump to add this in src/wifi/helper/yans-wifi-helper.cc. Grep for "Ascii".
Now if you want to know why it does not do this by default because it is so useful, I honestly can't remember but:
there is probably something related to the ns2 trace format that inspired this ascii format. Compatibility with existing tools might have been an issue
adding a packet id goes against the ns-3 philosophy of matching what real networks do