learning-gem5: Add a missing override

Change-Id: I9acebe6f3096b499fa2c69b6d757373431f63c71
Signed-off-by: Hoa Nguyen <hn@hnpl.org>

src/learning_gem5/part2/hello_object.hh

@@ -69,7 +69,7 @@ class HelloObject : public SimObject
      * SimObjects have been constructed. It is called after the user calls
      * simulate() for the first time.
      */
-    void startup();
+    void startup() override;
 };
 
 } // namespace gem5