Fixed setIcon with pixmap error

[j0yu]

⚡ Super quick one!

This patch fixes the following errors encountered as we try implement tk-multi-workfiles2 "File Open..." dialog tk-katana for Katana 3.1 (PyQt5):

• [ERROR python.root]: A TypeError occurred in "step_list_filter.py": setIcon(self, QIcon): argument 1 has unexpected type 'QPixmap'

  Traceback (most recent call last):
    File "/Volumes/wwfx/shotgun/test_project/joe/install/app_store/tk-multi-workfiles2/v0.11.10/python/tk_multi_workfiles/step_list_filter.py", line 154, in set_widgets_for_entity_type
      self._ensure_widgets_for_entity_type(linked_entity_type)
    File "/Volumes/wwfx/shotgun/test_project/joe/install/app_store/tk-multi-workfiles2/v0.11.10/python/tk_multi_workfiles/step_list_filter.py", line 223, in _ensure_widgets_for_entity_type
      widget.setIcon(pixmap)
  TypeError: setIcon(self, QIcon): argument 1 has unexpected type 'QPixmap'