small fix for 50d71d5ea

The error was:
```
Error: Python: Traceback (most recent call last):
  File "\blenderbim\bim\module\model\window.py", line 337, in execute
    element = blenderbim.core.root.assign_class(
NameError: name 'blenderbim' is not defined
```

src/blenderbim/blenderbim/bim/module/model/window.py

@@ -22,6 +22,7 @@ import json
 import bmesh
 import collections
 import ifcopenshell
+import blenderbim
 import blenderbim.tool as tool
 import blenderbim.core.geometry as core
 from ifcopenshell.api.geometry.add_window_representation import DEFAULT_PANEL_SCHEMAS