meta-flow: Add support for ONF OpenFlow 1.3 extension ONFOXM_ET_TCP_FLAGS.

Actually the draft extension says "TCP_FLAG", without an "S", but I think
that's just a typo.

ONF-JIRA: EXT-109
Signed-off-by: Ben Pfaff <blp@nicira.com>
Acked-by: Jarno Rajahalme <jrajahalme@nicira.com>