debian/control: add adduser to Depends of cephadm and ceph-common

debian/control: add adduser to Depends of cephadm and ceph-common

in `debian/ceph-common.postinst` and `debian/cephadm.postinst`, we
use `adduser --system` to create the system user when configuring
the corresponding package.

before this change, the dependency is not listed in the runtime
`Depends` section of ceph-common and cephadm.

in this change, the dependency is added. this is also suggested
by Securing Debian Manual, see
https://www.debian.org/doc/manuals/securing-debian-manual/bpp-lower-privs.en.html

Signed-off-by: Kefu Chai <tchaikov@gmail.com>