Fix build break in ofproto/tunnel.c for windows platform.

The breakage was introduced by commit: a36de779
("openvswitch: Userspace tunneling.").

Reported-by: Edwin Chiu <echiu@nicira.com>
Signed-off-by: Saurabh Shah <ssaurabh@vmware.com>
Acked-by: Nithin Raju <nithin@vmware.com>
Signed-off-by: Gurucharan Shetty <gshetty@nicira.com>