add NODE_EXTERN to node::Encode

[defunctzombie]

Working on making the bcrypt module work on windows. It uses the Encode function, however that is not exported.

See:
kelektiv/node.bcrypt.js#92 (comment)

Would be nice to have access to this function :)

[bnoordhuis]

@piscisaureus Does that change the ABI in a non-backwards compatible way?

[piscisaureus]

@bnoordhuis Nope.

[bnoordhuis]

Changed in 53f3b12.