Define variable to hold OVS header location.
Will make transition to properly installed OVS headers easier.

Change-Id: Id0e4bdc96b9ece5760b9a59a3b56974a584be3d6
Signed-off-by: Amit Bose <bose@noironetworks.com>
Merge "Define variable to hold OVS header location. Will make transition to properly installed OVS headers easier."