common/onion_decode: put final flag in onion_payload.

You can use rs->nextcase, but we don't always keep that around, so
keep a flag in onion_payload.

We'll use this in the "do we need to return a blinded error code"
patch.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

common/onion_encode.h

@@ -14,6 +14,8 @@ enum onion_payload_type {
 
 struct onion_payload {
 	enum onion_payload_type type;
+	/* Is this the final hop? */
+	bool final;
 
 	struct amount_msat amt_to_forward;
 	u32 outgoing_cltv;