Fix the following make W=1 kernel build warning:

  drivers/rtc/sysfs.c:115: warning: expecting prototype for rtc_sysfs_show_hctosys(). Prototype was for hctosys_show() instead

Signed-off-by: Yang Yingliang <yangyingliang@huawei.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Link: https://lore.kernel.org/r/20210517045901.3461800-1-yangyingliang@huawei.com