do not throw an error if `extendNodeEncodings()` is called again

Rationale:

* My package [`fidonet-jam`](https://www.npmjs.org/package/fidonet-jam) currently [uses](https://github.com/Mithgol/node-fidonet-jam/blob/8cc852e2e51dfcb68d79ab20989cf3efd07e9d41/fidonet-jam.js#L6) `require('iconv-lite').extendNodeEncodings()`

* My package [`simteconf`](https://www.npmjs.org/package/simteconf) currently [uses](https://github.com/Mithgol/simteconf/blob/4f5646eb445bcae7f06e028b0502bff358ab26b5/simteconf.js#L9) `require('iconv-lite').extendNodeEncodings()`

* One of my applications ([PhiDo](https://github.com/Mithgol/phido)) currently [uses](https://github.com/Mithgol/phido/blob/7f7516e9e21102b06a174cd1db55c3ecffd98a73/package.json#L14-17) both of these packages, though not their current versions. It should probably not throw an error on start even if I decide to upgrade them both.