Fix : Use process.memoryUsage() in /_health.

lib/xapi.js

@@ -400,7 +400,7 @@ class Xapi {
     status['mysql_uptime'] = await this.getMysqlUptime();
 
     if (Object.keys(req.query).length) {
-      status['process_memory_usage'] = process.memoryUsage;
+      status['process_memory_usage'] = process.memoryUsage();
       status['os_total_memory'] = os.totalmem();
       status['os_free_memory'] = os.freemem();
       status['os_load_average'] = os.loadavg();