fix import error in workflow export

Import the pickle module, which is required for workflow export
function.

Author: blakedewey

nipype/pipeline/engine/utils.py

@@ -30,6 +30,7 @@
     from funcsigs import signature
 import os
 import re
+import pickle
 import numpy as np
 from nipype.utils.misc import package_check
 from functools import reduce