datapath-windows: Fixed Release broken build.

The release configurations of the OVSEXT project were not compiling.
This was due to a warning that was treated as error. Fixed that.

I did not want to remove that variable, because it is used in an
ASSERT in that function.

Signed-off-by: Sorin Vinturis <svinturis@cloudbasesolutions.com>
Acked-by: Alin Gabriel Serdean <aserdean@cloudbasesolutions.com>