Fix build with Qt 5.11_beta3 (dropping qt5_use_modules)

Summary:
A change in Qt 5.11_beta3 leads to errors like:

CMake Error at src/lib/CMakeLists.txt:62 (qt5_use_modules):
  `Unknown CMake command "qt5_use_modules".`

Test Plan: Built fine.

Reviewers: #plasma, davidedmundson

Reviewed By: #plasma, davidedmundson

Differential Revision: https://phabricator.kde.org/D12186