qtsingleapplication: unbreak build with Qt4

[barracuda156]

Fixes the build broken by 2fb541e

[barracuda156]

Could we merge this?

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}