add da15fc2fa9c "perf tools: Disable parallelism for 'make clean'" to -stable

Hi Greg and other -stable maintainers

Please consider adding da15fc2fa9c (perf tools: Disable parallelism for 'make clean') to your -stable trees. Spurious build failures like that make it harder to do automatic tests of new -stable (and -stable-rc) versions.

Thanks, Rasmus