datapath-windows: move OVS_MESSAGE to Netlink.h

datapath-windows: move OVS_MESSAGE to Netlink.h

Moved the structure OVS_MESSAGE to Netlink.h.
This change is done for following reasons.

a. Patch 2 in this series provides a generic API in Netlink.c
for creating netlink message. That API needs OVS_MESSAGE.
Including Datapath.h in Netlink.c/h gives compilation error.

b. OVS_MESSAGE defines netlink messages hence moving it to
Netlink.h looked fine to me.

Signed-off-by: Ankur Sharma <ankursharma@vmware.com>
Acked-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Acked-by: Eitan Eliahu <eliahue@vmware.com>
Acked-by: Nithin Raju <nithin@vmware.com>
Acked-by: Samuel Ghinet <sghinet@cloudbasesolutions.com>
Tested-by: Ankur Sharma <ankursharma@vmware.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>