[grantleebuilder/plaintextmarkupbuilder] Fix for Qt older than 5.15.2
QString append(QStringView str) added with Qt 5.15.2 and kpimtext qt is requiring 5.15.0 so this will break build on some systems
QString append(QStringView str) added with Qt 5.15.2 and kpimtext qt is requiring 5.15.0 so this will break build on some systems