luci-app-hd-idle: depend on lsblk

Since the last refactoring, luci-app-hd-idle requires lsblk to enumerate
disk devices.

Fixes: #5527
Fixes: 55e725ab

 ("luci-app-hd-idle: Improve content shown by luci.")
Signed-off-by: Jo-Philipp Wich <jo@mein.io>