app/osd: fix dde-osd --SwitchMonitors didn't appear

tower: https://tower.im/projects/8f19f0bf0e754f0b82ef2c24bc230973/todos/e81e847c9b584b728b242edcba739c05/

Change-Id: Ifcbf39e830514e8b3b2e957de272c326434e3511