common/shutdown.cpp: Start user worker for CONFIG_BUILD_KERNEL

The userspace work queue is not started automatically by the system,
so start it when dispatching the shutdown command.

It would be best if there was an init function for shutdown.cpp but this
"lazy" init is just fine too.

platforms/common/shutdown.cpp

@@ -215,6 +215,11 @@ int px4_reboot_request(bool to_bootloader, uint32_t delay_us)
 		return 0;
 	}
 
+#ifdef CONFIG_BUILD_KERNEL
+	// Must start the worker as it is not automatically started by the system
+	work_usrstart();
+#endif
+
 	shutdown_args |= SHUTDOWN_ARG_REBOOT;
 
 	if (to_bootloader) {