sweep+input: add RequiredTxOut to inputs

This is needed to sweep second level HTLC transactions that are signed
using SINGLE|ANYONECANPAY, since the input and ouput must be aligned.

breacharbiter.go

@@ -866,6 +866,13 @@ func (bo *breachedOutput) OutPoint() *wire.OutPoint {
 	return &bo.outpoint
 }
 
+// RequiredTxOut returns a non-nil TxOut if input commits to a certain
+// transaction output. This is used in the SINGLE|ANYONECANPAY case to make
+// sure any presigned input is still valid by including the output.
+func (bo *breachedOutput) RequiredTxOut() *wire.TxOut {
+	return nil
+}
+
 // RequiredLockTime returns whether this input commits to a tx locktime that
 // must be used in the transaction including it.
 func (bo *breachedOutput) RequiredLockTime() (uint32, bool) {