Mark single-argument constructors "explicit"

src/qt/transactiontablemodel.cpp

@@ -58,7 +58,7 @@ struct TxLessThan
 class TransactionTablePriv
 {
 public:
-    TransactionTablePriv(TransactionTableModel *_parent) :
+    explicit TransactionTablePriv(TransactionTableModel *_parent) :
         parent(_parent)
     {
     }