Minimal changes to comply with explicit CMutableTransaction -> CTranaction conversion.

This commit makes the minimal changes necessary to fix compilation once CTransaction(const CMutableTransaction &tx) is made explicit. In each case an explicit call `CTransaction(...)` was added. Shouldn't affect behaviour or performance.

src/script/sign.cpp

@@ -509,7 +509,7 @@ bool IsSolvable(const SigningProvider& provider, const CScript& script)
     return false;
 }
 
-PartiallySignedTransaction::PartiallySignedTransaction(const CTransaction& tx) : tx(tx)
+PartiallySignedTransaction::PartiallySignedTransaction(const CMutableTransaction& tx) : tx(tx)
 {
     inputs.resize(tx.vin.size());
     outputs.resize(tx.vout.size());