Fix importing zlib for commonjs modules

Fixes isaacs/node-tar#422

PR-URL: #31
Credit: @Goodwine
Close: #31
Reviewed-by: @isaacs

Author: Goodwine

src/index.ts

@@ -1,7 +1,7 @@
 import assert from 'assert'
 import { Buffer } from 'buffer'
 import { Minipass } from 'minipass'
-import realZlib from 'zlib'
+import * as realZlib from 'zlib'
 import { constants } from './constants.js'
 export { constants } from './constants.js'