channel.psbt: make non-const

We update it in the next patch, which technically breaks this contract.
So we shouldn't have the contract of const on this in the first place
then.

lightningd/channel.h

@@ -150,7 +150,7 @@ struct channel {
 	u64 rr_number;
 
 	/* PSBT, for v2 channels. Saved until it's sent */
-	const struct wally_psbt *psbt;
+	struct wally_psbt *psbt;
 
 	/* Stashed pps, saved until channeld is started.
 	 * Needed only for v2 channel open flow */

lightningd/dual_open_control.c

@@ -43,7 +43,9 @@ static void handle_signed_psbt(struct lightningd *ld,
 			       struct commit_rcvd *rcvd)
 {
 	/* Now that we've got the signed PSBT, save it */
-	rcvd->channel->psbt = tal_steal(rcvd->channel, psbt);
+	rcvd->channel->psbt =
+		tal_steal(rcvd->channel,
+			  cast_const(struct wally_psbt *, psbt));
 	wallet_channel_save(ld->wallet, rcvd->channel);
 
 	channel_watch_funding(ld, rcvd->channel);