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: https://github.com/nodejs/node/pull/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>