index.d.ts: fix lingui:extract blowing up on Unexpected token function

[himdel]

Running npm run gettext:extract:

#: src/containers/namespace-detail/namespace-detail.tsx:158
Cannot process file /home/himdel/ansible-hub-ui/src/index.d.ts: Unexpected token (45:25)

  43 |       identifyApp: (s: string) => void;
  44 |       init: () => void;
> 45 |       on: (s: string, f: function) => void;
    |                          ^
  46 |     };
  47 |   };
  48 | }

(it doesn't fail with a non-zero exit status code either :( )

This was casued by #1463 , looks like function is not supported everywhere, () => void works.