revalidator: Fix a build issue.

Commit acaa8dac490cc6c36026d45f3010ec75f38ee142 (revalidator: Eliminate
duplicate flow handling.) introduced a build error. This fixes the bug.

Reported-by: Jarno Rajahalme <jrajahalme@nicira.com>
Signed-off-by: Joe Stringer <joestringer@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>