netlink-socket: Set socket pid number in NL message on Windows.

The pid must be set in the NL header as the driver checks it against the pid in
the instance paired with the socket.

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