more clang-tidy fixes after re-running it with added settings

examples/plugins/helloplugin.cpp

@@ -12,7 +12,7 @@ namespace LAMMPS_NS {
 class Hello : public Command {
  public:
   Hello(class LAMMPS *lmp) : Command(lmp){};
-  void command(int, char **);
+  void command(int, char **) override;
 };
 }    // namespace LAMMPS_NS