Avoid unused variable definition
Fix this warning:
/home/test/kde/src/kconfig/src/core/kdesktopfile.cpp: In member function ‘bool KDesktopFile::tryExec() const’:
/home/test/kde/src/kconfig/src/core/kdesktopfile.cpp:273:20: warning: unused variable ‘su’ [-Wunused-variable]
273 | if (const bool su = d->desktopGroup.readEntry("X-KDE-SubstituteUID", false)) {
| ^~
(Not a C++ programmer here, but I guess this is optimized out by the compiler anyway, is just annoying to see a warning...)