Append 'useDKIM’, 'close' function's return-type annotation

For fix this compile error.
```
> tsc --noImplicitAny nodemailer/nodemailer.d.ts
nodemailer.d.ts(20,2): error TS7010: 'useDKIM', which lacks return-type annotation, implicitly has an 'any' return type.
nodemailer.d.ts(21,2): error TS7010: 'close', which lacks return-type annotation, implicitly has an 'any' return type.
```

[‘useDKIM’](https://github.com/andris9/Nodemailer/blob/master/lib/transport.js#L89) function only set argument value.
[‘close’](https://github.com/andris9/Nodemailer/blob/master/lib/transport.js#L100) function return callback result.