shadow: add runtime dependency on base-passwd

shadow: add runtime dependency on base-passwd

This fixes an issue where the shadow package was sometimes getting
installed before base-passwd during rootfs construction with Smart,
which meant the postinstall scripts could not find /etc/passwd and
/etc/group and failed as a result.

(From OE-Core rev: 6a660d39250cc635f7c1b6ae437f8fb40b7aec12)

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