Avoids calling ffi.dlopen(None) on Windows
Avoids calling ffi.dlopen(None) on Windows

ffi.dlopen(None) cannot work on Windows and it is a
known issue [1]. This patch avoids this call on Windows
hosts.

[1] https://bugs.python.org/issue23606

Change-Id: I44436146cdc402087f33f4b99054176134ec5b81
Closes-Bug: #1788025

openstack/oslo.privsep