benchmarks/stress-ng: Updating to version 0.08.13

Changes: http://kernel.ubuntu.com/git/cking/stress-ng.git/commit/?id=ce8f7d55523c752f65bbbfbdeee4e7cecf190313
Changes: http://kernel.ubuntu.com/git/cking/stress-ng.git/commit/?id=c31cea18d68677a01e879950be19b7d6b863b2d3

Approved by: lme (mentor)
Approved by: olivier (mentor)
Differential Revision: https://reviews.freebsd.org/D12204