Xz.git: Fix Linux Landlock Feature Test in Autotools

A few reasons have been previously discussed: plenty of diff viewers highlight non-standard characters being a big one, the other bit being if someone does see this error it seems entirely plausible as a mistake.

Choosing something atypical in a way that is clearly trying to be invisible rather than being an accident will draw a lot more attention than “oops I must have typo’d it”