test: fix warning in n-api reference test

Add cast to avoid warning during build of addon.

PR-URL: https://github.com/nodejs/node/pull/12730
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

test/addons-napi/test_reference/test_reference.c

@@ -60,10 +60,10 @@ napi_value CheckExternal(napi_env env, napi_callback_info info) {
 
   NAPI_ASSERT(env, argtype == napi_external, "Expected an external value.")
 
-  int* data;
+  void* data;
   NAPI_CALL(env, napi_get_value_external(env, arg, &data));
 
-  NAPI_ASSERT(env, data != NULL && *data == test_value,
+  NAPI_ASSERT(env, data != NULL && *(int*)data == test_value,
     "An external data value of 1 was expected.")
 
   return NULL;