Add missing override keyword:

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

src/test/ledger/CashDiff_test.cpp

@@ -91,7 +91,7 @@ class CashDiff_test : public beast::unit_test::suite
     }
 
 public:
-    void run ()
+    void run () override
     {
         testDust();
     }