n-api: fix -Wmismatched-tags compiler warning

`napi_env__` was declared as a struct in one place and a class in
another.

PR-URL: https://github.com/nodejs/node/pull/12333


Reviewed-By: Anna Henningsen <anna@addaleax.net>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Daniel Bevenius <daniel.bevenius@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>