funding tx: include segwit marker + flag in fee calculation

Noticed an off by one error when running tests for dual-funding;
we're not including the two 'header' segwit bytes in our weight
calculations.

common/wallet_tx.c

@@ -184,7 +184,8 @@ struct command_result *wtx_from_utxos(struct wallet_tx *tx,
 
 	/* The transaction has `tal_count(tx.utxos)` inputs and one output */
 	/* (version + in count + out count + locktime)  (index + value + script length) */
-	weight = 4 * (4 + 1 + 1 + 4) + 4 * (8 + 1 + out_len);
+	/* + segwit marker + flag */
+	weight = 4 * (4 + 1 + 1 + 4) + 4 * (8 + 1 + out_len) + 1 + 1;
 	for (size_t i = 0; i < tal_count(utxos); i++) {
 		if (!*utxos[i]->blockheight || *utxos[i]->blockheight > maxheight) {
 			tal_arr_remove(&utxos, i);

devtools/mkfunding.c

@@ -90,6 +90,8 @@ int main(int argc, char *argv[])
 
 	/* nVersion, input count, output count, nLocktime */
 	weight = 4 * (4 + 1 + 1 + 4);
+	/* Add segwit fields: marker + flag */
+	weight += 1 + 1;
 	/* Single output: Satoshis, script length, p2wsh. */
 	weight += 4 * (8 + 1 + BITCOIN_SCRIPTPUBKEY_P2WSH_LEN);
 	/* Single input: txid, index, scriptlen, nSequence */

wallet/wallet.c

@@ -289,6 +289,9 @@ static const struct utxo **wallet_select(const tal_t *ctx, struct wallet *w,
 	/* version, input count, output count, locktime */
 	weight = (4 + 1 + 1 + 4) * 4;
 
+	/* Add segwit fields: marker + flag */
+	weight += 1 + 1;
+
 	/* The main output: amount, len, scriptpubkey */
 	weight += (8 + 1 + outscriptlen) * 4;