Fix winapi initialization on Python 2.5 on Windows

This regression (apparently introduced in 13892788c315bc2) was uncovered during testing #193/#230.

Rudimentary test result below. Doesn't seem to break 2.6/2.7 (though more eyes looking at `get_thread_ident()` might not hurt).

```
PYTHON: C:\Python25 -- LIBRARY: tip.paramiko -- TEST: agent_init.py
  Traceback: AttributeError: 'module' object has no attribute 'USHORT'
PYTHON: C:\Python26 -- LIBRARY: tip.paramiko -- TEST: agent_init.py
PYTHON: C:\Python27 -- LIBRARY: tip.paramiko -- TEST: agent_init.py
PYTHON: C:\Python25 -- LIBRARY: paramiko-git-akx -- TEST: agent_init.py
PYTHON: C:\Python26 -- LIBRARY: paramiko-git-akx -- TEST: agent_init.py
PYTHON: C:\Python27 -- LIBRARY: paramiko-git-akx -- TEST: agent_init.py
```