do not fail build if unable to relabel selinux content

If the failure is a problem, the build will fail later.  But it is
possible that this is not a fatal issue and we should let things go
forward.  (a filesystem mounted with context=something in permissive
would cause chcon to fail, but the build to work)
Merge pull request #2348 from eparis/selinux-permissive

do not fail build if unable to relabel selinux content