fix(NODE-6108): allow building from source on latest Node.js 20.x

See nodejs/node#52229 – addons using certain features
of the node-addon-api package have been broken by an upstream Node.js change if
they were setting `NAPI_EXPERIMENTAL`, which `kerberos` is doing.

Author: addaleax

src/kerberos.h

@@ -7,6 +7,7 @@
 #define NAPI_VERSION 6
 // as an experimental feature (that has not been changed since then).
 #define NAPI_EXPERIMENTAL
+#define NODE_API_EXPERIMENTAL_NOGC_ENV_OPT_OUT
 
 #include <napi.h>
 #include "kerberos_common.h"