libwally: update to 0.6.8.

This fixes block parsing on testnet; specifically, non-standard tx versions.

We hit a type bug in libwally (wallt_get_secp_context()) which I had to
work around for the moment, and the updated libsecp adds an optional hash
function arg to the ECDH function.

Fixes: #2563
Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

devtools/gossipwith.c

@@ -77,7 +77,7 @@ struct secret *hsm_do_ecdh(const tal_t *ctx, const struct pubkey *point)
 {
 	struct secret *ss = tal(ctx, struct secret);
 	if (secp256k1_ecdh(secp256k1_ctx, ss->data, &point->pubkey,
-			   notsosecret.data) != 1)
+			   notsosecret.data, NULL, NULL) != 1)
 		return tal_free(ss);
 	return ss;
 }