n-api: fix -Wmismatched-tags compiler warning

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

PR-URL: #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>

src/node_api.cc

@@ -22,8 +22,7 @@ napi_status napi_set_last_error(napi_env env, napi_status error_code,
                                 void* engine_reserved = nullptr);
 void napi_clear_last_error(napi_env env);
 
-class napi_env__ {
- public:
+struct napi_env__ {
   explicit napi_env__(v8::Isolate* _isolate): isolate(_isolate),
       has_instance_available(true), last_error() {}
   ~napi_env__() {