n-api: change #ifdef to #if in node_api_types

Currently, there are a number of compiler warnings like the following:
In file included from ../src/node_api.h:11:
../src/node_api_types.h:13:21:x
 warning: extra tokens at end of #ifdef directive [-Wextra-tokens]
 #ifdef NAPI_VERSION >= 4

This commit changes the #ifdef macros to #if.

PR-URL: #25635
Reviewed-By: Ben Noordhuis <info@bnoordhuis.nl>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Michael Dawson <michael_dawson@ca.ibm.com>
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Anna Henningsen <anna@addaleax.net>

src/node_api_types.h

@@ -10,7 +10,7 @@ typedef struct napi_async_work__* napi_async_work;
 typedef struct napi_threadsafe_function__* napi_threadsafe_function;
 #endif  // NAPI_VERSION >= 4
 
-#ifdef NAPI_VERSION >= 4
+#if NAPI_VERSION >= 4
 typedef enum {
   napi_tsfn_release,
   napi_tsfn_abort
@@ -27,7 +27,7 @@ typedef void (*napi_async_execute_callback)(napi_env env,
 typedef void (*napi_async_complete_callback)(napi_env env,
                                              napi_status status,
                                              void* data);
-#ifdef NAPI_VERSION >= 4
+#if NAPI_VERSION >= 4
 typedef void (*napi_threadsafe_function_call_js)(napi_env env,
                                                  napi_value js_callback,
                                                  void* context,