netlink-socket: Use poll_immediate_wake() on Windows.

We have not yet tested the wakup via pending IRP functionality on
Windows yet. Hence we use poll_immediate_wake().

Signed-off-by: Nithin Raju <nithin@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Eitan Eliahu <eliahue@vmware.com>