psbt: add PSBTv2 global tx fields

When decomposing a transaction into a PSBTv2, the tx version and
locktime need to be stored in their respective global fields. Add those
fields and fill them when constructing.

src/psbt.cpp

@@ -16,6 +16,8 @@ using common::PSBTError;
 
 PartiallySignedTransaction::PartiallySignedTransaction(const CMutableTransaction& tx) : tx(tx)
 {
+    tx_version = tx.version;
+    fallback_locktime = tx.nLockTime;
     inputs.reserve(tx.vin.size());
     for (const CTxIn& input : tx.vin) {
         inputs.emplace_back(GetVersion(), input.prevout.hash, input.prevout.n, input.nSequence);