Add missing override keyword:

* Enable the `suggest-override` warning for gcc
* Fix all functions that were flagged by that warning

src/ripple/protocol/TxFormats.h

@@ -65,7 +65,7 @@ enum TxType
 class TxFormats : public KnownFormats <TxType>
 {
 private:
-    void addCommonFields (Item& item);
+    void addCommonFields (Item& item) override;
 
 public:
     /** Create the object.