Build fails with Qt 5.12

[lgbaldoni]

On openSUSE Leap 15.2 with Qt 5.12.7, see

[ 27%] Building CXX object src/CMakeFiles/haruna.dir/application.cpp.o
cd /home/abuild/rpmbuild/BUILD/haruna-0.3.0/build/src && /usr/bin/c++  -DKCOREADDONS_LIB -DQT_CORE_LIB -DQT_DBUS_LIB -DQT_GUI_LIB -DQT_NETWORK_LIB -DQT_NO_DEBUG -DQT_QML_DEBUG -DQT_QML_LIB -DQT_QUICKCONTROLS2_LIB -DQT_QUICK_LIB -DQT_WIDGETS_LIB -DQT_XML_LIB -I/home/abuild/rpmbuild/BUILD/haruna-0.3.0/build/src -I/home/abuild/rpmbuild/BUILD/haruna-0.3.0/src -I/home/abuild/rpmbuild/BUILD/haruna-0.3.0/build/src/haruna_autogen/include -isystem /usr/include/qt5 -isystem /usr/include/qt5/QtCore -isystem /usr/lib64/qt5/mkspecs/linux-g++ -isystem /usr/include/qt5/QtQuick -isystem /usr/include/qt5/QtQml -isystem /usr/include/qt5/QtNetwork -isystem /usr/include/qt5/QtGui -isystem /usr/include/qt5/QtQuickControls2 -isystem /usr/include/KF5/KConfigGui -isystem /usr/include/KF5 -isystem /usr/include/qt5/QtXml -isystem /usr/include/KF5/KConfigCore -isystem /usr/include/KF5/KCoreAddons -isystem /usr/include/KF5/KFileMetaData -isystem /usr/include/KF5/KI18n -isystem /usr/include/KF5/KXmlGui -isystem /usr/include/qt5/QtWidgets -isystem /usr/include/KF5/KConfigWidgets -isystem /usr/include/KF5/KCodecs -isystem /usr/include/KF5/KWidgetsAddons -isystem /usr/include/KF5/KAuth -isystem /usr/include/qt5/QtDBus  -fmessage-length=0 -grecord-gcc-switches -O2 -Wall -D_FORTIFY_SOURCE=2 -fstack-protector-strong -funwind-tables -fasynchronous-unwind-tables -fstack-clash-protection -g -DNDEBUG -O2 -g -DNDEBUG   -fPIC -std=gnu++14 -o CMakeFiles/haruna.dir/application.cpp.o -c /home/abuild/rpmbuild/BUILD/haruna-0.3.0/src/application.cpp
/home/abuild/rpmbuild/BUILD/haruna-0.3.0/src/application.cpp: In member function 'void Application::aboutApplication()':
/home/abuild/rpmbuild/BUILD/haruna-0.3.0/src/application.cpp:245:30: error: aggregate 'QPointer<QDialog> dialog' has incomplete type and cannot be defined
     static QPointer<QDialog> dialog;
                              ^~~~~~