package/lxc: cgroups: initialize cpuset properly

The tests.package.test_lxc.TestLxc failure on gitlab
is similar to the issue reported by [1] and fixed by [2].

Fixes:
https://gitlab.com/buildroot.org/buildroot/-/jobs/454255988

[1] https://github.com/NixOS/nixpkgs/issues/75467#issuecomment-569386159
[2] https://github.com/lxc/lxc/pull/3109



Signed-off-by: Romain Naour <romain.naour@smile.fr>
Cc: Jérôme Pouiller <jezz@sysmic.org>
Cc: Patrick Havelange <patrick.havelange@essensium.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>