Show all headers in qtc6 + use target_sources

kmail/grammarplugins/languagetool/tests/CMakeLists.txt

 # SPDX-FileCopyrightText: 2019-2022 Laurent Montel <montel@kde.org>
 # SPDX-License-Identifier: BSD-3-Clause
 
-set(languagetool_gui_SRCS main.cpp languagetoolwidget.cpp)
-add_executable(languagetool_gui ${languagetool_gui_SRCS})
+add_executable(languagetool_gui)
+target_sources(languagetool_gui PRIVATE main.cpp languagetoolwidget.cpp languagetoolwidget.h)
 target_link_libraries(languagetool_gui
     KF5::I18n kmaillanguagetool grammarcommon
 )