Summary:
Provide QProcess arguments as specified by documentation. Apparently,
with Qt 5.6 QProcess stopped to accept the previously used single string
argument. This fixes detection of the system's qthelp documentation for
Qt.

Test Plan: Manual testing.

Reviewers: apol, #kdevelop, kfunk

Reviewed By: apol, #kdevelop, kfunk

Subscribers: kfunk, apol, kdevelop-devel

Differential Revision: https://phabricator.kde.org/D2065