VFolderMenu: Add const to local QString to fix compilation with Qt 6.7
C++ ternary on line 601 does not like that QStringBuilders refer to strings of different constness.
The error itself:
/home/ratijas/kde/src6/kservice/src/sycoca/vfolder_menu.cpp: In member function ‘void VFolderMenu::mergeMenus(QDomElement&, QString&)’:
/home/ratijas/kde/src6/kservice/src/sycoca/vfolder_menu.cpp:601:56: error: operands to ‘?:’ have different types ‘QStringBuilder<QString&, const QString&>’ and ‘QStringBuilder<const QString&, const QString&>’
601 | const QString fileToAdd = relative ? dir + file : menuDir + file;
| ~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~