socket-util: af_inet_ioctl() for Windows.

There is no direct mapping for the ioctl function in
Windows.  As of now, af_inet_ioctl() is being used for Linux
and BSD. So, don't try to compile it for Windows.

Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>
Acked-by: Ben Pfaff <blp@nicira.com>