src: guard exit label when inspector disabled

Currently, when configured --without-inspector the following warning
will be generated:
../src/node.cc:859:1: warning: unused label 'exit' [-Wunused-label]
exit:

This commit adds a guard to exclude the label when there is no
inspector support.

PR-URL: nodejs#26801
Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: James M Snell <jasnell@gmail.com>
Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Yongsheng Zhang <zyszys98@gmail.com>

src/node.cc

@@ -856,7 +856,9 @@ inline int StartNodeWithIsolate(Isolate* isolate,
 
   WaitForInspectorDisconnect(&env);
 
+#if HAVE_INSPECTOR && NODE_USE_V8_PLATFORM
 exit:
+#endif
   env.set_can_call_into_js(false);
   env.stop_sub_worker_contexts();
   uv_tty_reset_mode();