fix [rare] concurrent build issue:

[709/779] subst: headless/hardev_common.sh.in -> build/headless/hardev_common_waf.sh
[710/779] hardev_common_waf.sh: -> build/headless/hardev_common_waf.sh
chmod: cannot access `headless/hardev_common_waf.sh': No such file or directory
add explicit chmod for good measure