Fix pygmalion theme: add 'autoload "add-zsh-hook"'

Currently with `ZSH_THEME="pygmalion"` in my `~/.zshrc`, I get the following error message:
```prompt_setup_pygmalion:12: command not found: add-zsh-hook```

This PR fixes it, but I'm not sure, whether I placed the `autoload` command in the correct file and line.