KDesktopFile::isAuthorizedDesktopFile: reduce warning to info a log

If a desktop file would be on your desktop without executable, it gets an emblem-important. And plasmashell output is spammed with:

kf.config.core: Access to ".desktop" ' denied, not owned by root, executable flag not set.

This lowers this warning to info.

This function is used in KFileItem::overlays