cpuidle: fix a build error when compiling haltpoll into module

hulk inclusion
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I3ZURN
CVE: NA

--------

Kernel build would fail in case of CONFIG_HALTPOLL_CPUIDLE=m, caused by
haltpoll_switch_governor() not marked as an exported symbol. Fix this
by complementing the EXPORT_SYMBOL statement.

Fixes: 97c22788 ("cpuidle: fix container_of err in cpuidle_device and cpuidle_driver")
Signed-off-by: NGONG, Ruiqi <gongruiqi1@huawei.com>
Cc: Jiajun Chen <chenjiajun8@huawei.com>
Reviewed-by: NXie XiuQi <xiexiuqi@huawei.com>
Reviewed-by: NKeqian Zhu <zhukeqian1@huawei.com>
Signed-off-by: NCheng Jian <cj.chengjian@huawei.com>

drivers/cpuidle/driver.c

@@ -259,6 +259,7 @@ void haltpoll_switch_governor(struct cpuidle_driver *drv)
 		mutex_unlock(&cpuidle_lock);
 	}
 }
+EXPORT_SYMBOL_GPL(haltpoll_switch_governor);
 
 /**
  * cpuidle_register_driver - registers a driver