Fixed the ImportError when importing gxf in gdb from fresh install

From a fresh install (python3 setup.py install), when importing inside gdb, I got the following ```ImportError```

```
(gdb) python import gxf.extensions
Traceback (most recent call last):
  File "<string>", line 1, in <module>
ImportError: No module named 'gxf.extensions'
Error while executing Python code.
```