fix tracecmd.py

[carverdamien]

Traceback (most recent call last):
  File "tracecmd.py", line 251, in <module>
    print("\t%s" % (ev))
  File "tracecmd.py", line 66, in __str__
    (self.ts/1000000000, self.ts%1000000000, self.cpu, self.name,
  File "tracecmd.py", line 46, in _get
    value = func(self)
  File "tracecmd.py", line 91, in name
    return event_format_name_get(self._format)
NameError: global name 'event_format_name_get' is not defined

[alxhoff]

Cheers @carverdamien