odp-util: Make it harder to forget to update ODPUTIL_FLOW_KEY_BYTES.

We've forgotten more than once.  Most updates coincide with adding
new flow members, so a build assertion on FLOW_WC_SEQ seems appropriate.

Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Andy Zhou <azhou@nicira.com>