Declare single-argument (non-converting) constructors "explicit"

In order to avoid unintended implicit conversions.

src/script/interpreter.h

@@ -114,7 +114,7 @@ struct PrecomputedTransactionData
 {
     uint256 hashPrevouts, hashSequence, hashOutputs;
 
-    PrecomputedTransactionData(const CTransaction& tx);
+    explicit PrecomputedTransactionData(const CTransaction& tx);
 };
 
 enum SigVersion