sstate.bbclass: Check file ownership before doing 'touch -a'

In contrast to 'touching' a file without parameters 'touch -a' can only be
performed by the file owner. In case of a shared sstate-cache served
locally (e.g. over NFS),  where the user running bitbake is not the
owner of sstate-cache files, even if he has write access on group level,
the sstate_unpack_package task will fail.
Checking if the file is owned by the user before attempting to run
'touch -a' on it solves this.

Signed-off-by: Norman Stetter <norman.stetter@garz-fricke.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>