api: ipsec: add missing IS_INBOUND flag.

External IKE daemons need to be able to flag an SA as inbound (just as
the included ike plugin does). This commit adds this flag to the API.
This change is backward bug-compatible as not setting the flag (old
clients) continues to mean all SAs are created as outbound and fib nodes
are created for them. The addition of this flag inhibits this forwarding
node creation as well as properly flagging the SA as inbound.

Ticket: VPP-1845
Type: fix
Signed-off-by: Christian Hopps <chopps@labn.net>
Change-Id: I195e32c430e51155fac2d9f33671e06ef42a3f7f