cal: fix missing import

dstat_interface/experiments/cal.py

@@ -29,7 +29,7 @@ import serial
 logger = logging.getLogger("dstat.experiments.cal")
 
 import state
-from experiments.experiment_template import Experiment
+from experiments.experiment_template import Experiment, dstat_logger
 
 def measure_offset(time):
     gain_trim_table = [None, 'r100_trim', 'r3k_trim', 'r30k_trim', 'r300k_trim',