Remove several warnings when building the project

Signed-off-by: Yihe Li <winmikedows@hotmail.com>

launcher/NullInstance.h

@@ -70,7 +70,7 @@ class NullInstance : public BaseInstance {
         return out;
     }
     QString modsRoot() const override { return QString(); }
-    void updateRuntimeContext()
+    void updateRuntimeContext() override
     {
         // NOOP
     }