gyp: fix gypfile name in index.js

Refs: nodejs/node-addon-api#643
PR-URL: nodejs/node-addon-api#658
Reviewed-By: Nicola Del Gobbo <nicoladelgobbo@gmail.com>
Reviewed-By: Gabriel Schulhof <gabriel.schulhof@intel.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>

index.js

@@ -2,7 +2,7 @@ const path = require('path');
 
 module.exports = {
   include: `"${__dirname}"`,
-  gyp: path.join(__dirname, 'nothing.gyp:nothing'),
+  gyp: path.join(__dirname, 'node_api.gyp:nothing'),
   isNodeApiBuiltin: true,
   needsFlag: false
 };