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

In order to avoid unintended implicit conversions.

src/qt/overviewpage.cpp

@@ -25,7 +25,7 @@ class TxViewDelegate : public QAbstractItemDelegate
 {
     Q_OBJECT
 public:
-    TxViewDelegate(const PlatformStyle *_platformStyle, QObject *parent=nullptr):
+    explicit TxViewDelegate(const PlatformStyle *_platformStyle, QObject *parent=nullptr):
         QAbstractItemDelegate(parent), unit(BitcoinUnits::BTC),
         platformStyle(_platformStyle)
     {