[plasmalnf] Older Qt doesn't have -> for iterator

With Qt 5.15.2 (and clang), `k->first` works, but this breaks
with Qt 5.11 (and gcc), this is not available and the dereference
  must be written differently, `(*k).first`.

src/modules/plasmalnf/ThemeInfo.cpp

@@ -185,7 +185,7 @@ ThemesModel::setThemeImage( const QMap< QString, QString >& images )
         QSignalBlocker b( this );
         for ( auto k = images.constKeyValueBegin(); k != images.constKeyValueEnd(); ++k )
         {
-            setThemeImage( k->first, k->second );
+            setThemeImage( (*k).first, (*k).second );
         }
     }
     emit dataChanged( index( 0, 0 ), index( m_themes->count() - 1 ), { ImageRole } );