d/control: add transitional zfs-dbg package

d/control: add transitional zfs-dbg package

suggesting the new -dbgsym packages without having a strict dependency
on a specific version of the library packages, like the old no longer
built zfs-dbg package had.

this commit can be reverted after the package has been uploaded once, or
kept for one release cycle if we might do the -dbgsym migration in
oldstable as well to avoid the oldstable zfs-dbg package version
overtaking the transitional one here.

Signed-off-by: Fabian Grünbichler <f.gruenbichler@proxmox.com>