Fix missing audio duration in details view

Enabling the 'duration' column in a folder with audio files did only show empty information. Properly initializing the QTime object fixes this.

 Testing Done

    Find a folder with audio files
    Make sure that Baloo has indexed this folder
    Open this folder in Dolphin, detail view, enable the Audio/Duration column
    Unpatched Dolphin does not display audio duration, patched Dolphin does

Tested on Gentoo x86_64 with Qt 5.5.1, KF 5.21.0 and Arch x86_64 with Qt 5.6.0 and KF 5.21.0.

REVIEW: 127799