win: fix custom actions for WiX older than 3.9

This is a port of joyent/node@e192f61514cd1500de895cb10128d412f842d7d0 .

Original commit message:

  Older WiX versions included a header with extern "C" declaration,
  hence the custom action source must be C++.

Reviewed-By: João Reis <reis@janeasystems.com>
  PR-URL: https://github.com/joyent/node/pull/25569

PR-URL: https://github.com/nodejs/node/pull/2365


Reviewed-By: Alexis Campailla <alexis@janeasystems.com>