datapath-windows: Update ovsext.sln to properly build under "Win8 Debug".

"Win8 Debug" solution configuration  currently builds in fact under
"Win8.1 Debug".

This patch updates the configuration to properly build under "Win8 Debug".

Reported-at: https://github.com/openvswitch/ovs-issues/issues/7
Signed-off-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Nithin Raju <nithin@vmware.com>