Fixed os.getppid() on windows

when exc openerp-gevent to start odoo, it will raise :

2014-06-10 17:21:33,051 668 [1;32m[1;49mINFO[0m ? openerp.service.server: Evented Service (longpolling) running on 0.0.0.0:8072
Traceback (most recent call last):
  File "D:\GreenOdoo\runtime\python\lib\gevent\greenlet.py", line 327, in run
    result = self._run(*self.args, **self.kwargs)
  File "D:\GreenOdoo\source\openerp\service\server.py", line 368, in watch_parent
    ppid = os.getppid()
AttributeError: 'module' object has no attribute 'getppid'
<Greenlet at 0x457a120: <bound method GeventServer.watch_parent of <openerp.service.server.GeventServer object at 0x040C1310>>> failed with AttributeError