Add missing override keyword:

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

src/ripple/rpc/impl/ServerHandlerImp.h

@@ -121,7 +121,7 @@ public:
     //
 
     void
-    onStop();
+    onStop() override;
 
     //
     // Handler

src/ripple/rpc/impl/WSInfoSub.h

@@ -68,7 +68,7 @@ public:
     }
 
     void
-    send(Json::Value const& jv, bool)
+    send(Json::Value const& jv, bool) override
     {
         auto sp = ws_.lock();
         if(! sp)