trace: add dpkt module

dpkt is a module which is used by 'trace.py tcpdump' and 'trace.py
list --tcpdump' to convert captured packets to PCAP format.

This change is about making trace.py work out of the box and require
no manual work of installing the dependency.

 Source:  https://code.google.com/p/dpkt/
 Version: 1.8
 License: 3-clause BSD
 Size:    94K (zipped)
trace: import dpkt unconditionally