Changes

sysmocom_sjs2: Make sure 'Const' is imported

  File "/crypt/space/home/laforge/projects/git/pysim/pySim/sysmocom_sja2.py", line 180, in __init__
    self._construct = Struct(Const(b'\x82'), 'time_unit'/self.TimeUnit, 'value'/Int8ub,
                             ^^^^^
NameError: name 'Const' is not defined

Change-Id: If34a48e349680ef84e68a4a1a19dde536ecda0e6