buildhistory: don't output ownership for the sysroot

As the sysroot isn't ran inside pseudo the ownership is whoever is running the
builds.  In a setup where multiple builders all contribute to a shared
buildhistory writing the ownership data isn't useful, so just replace it with "-
-".

(From OE-Core rev: fadb7ae78876a7cf25c48481ff4ed3131e53415f)

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>