[BUG] CMakeLists.txt gernerated by cpp-qt-client is missing Qt5:.Gui dependency / linking

Created by: rejuce

Bug Report Checklist

tried with Qt5.14, Qt5.15, with gcc7 under Linux / ubunut 20.04

• What's the actual output vs expected output? CMake should build, but fails with QDesktopServices.h can not be found. This header depens on Qt5:Gui which is not listed/linked in the autogernated CMakeLists.txt

openapi-generator version

5.3.1

Suggest a fix

add find_package(Qt5 COMPONENTS Gui REQUIRED) and Qt5::Gui under target_link_libraries in the generated CMakeLists.txt