classes/package_tar: fix file ownership within tarballs

The fakeroot flag wasn't correctly set leading to files within the
output tarballs having uid/gid values from the build host instead of the
desired target permissions.

Signed-off-by: Paul Eggleton <paul.eggleton@linux.intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>