fix: add missing import

Author: alexsavio

nipype/pipeline/engine.py

@@ -53,7 +53,7 @@
                                Undefined, TraitedSpec, DynamicTraitedSpec,
                                Bunch, InterfaceResult, md5, Interface,
                                TraitDictObject, TraitListObject, isdefined)
-from ..utils.misc import getsource, create_function_from_source
+from ..utils.misc import getsource, create_function_from_source, flatten
 from ..utils.filemanip import (save_json, FileNotFoundError,
                                filename_to_list, list_to_filename,
                                copyfiles, fnames_presuffix, loadpkl,