Backport patch to fix the build of KWin 5.

The error looks like this:

/wrkdirs/usr/ports/x11-wm/kde5-kwin/work/kwin-5.2.0/kcmkwin/kwintabbox/layoutpreview.cpp:135:66: error: redefinition of 's' with a different type: 'const QString' vs 'const auto'

if (const auto s = KMimeTypeTrader::self()->preferredService(QStringLiteral("inode/directory"))) {
                                                                 ^

/usr/local/include/qt5/QtCore/qstring.h:170:23: note: expanded from macro 'QStringLiteral'

const QString s(holder); \
              ^