Don't depend on libwbclient-sssd, as it doesn't exist anymore

libwbclient-sssd has been dropped on sssd v2.5.x, which is already on Debian testing. For this reason, freeipa-client-samba shouldn't depend on it anymore.

I decided to play it safe and depend on libwbclient-dev, which is built by samba. It doesn't seem like freeipa depends directly on the library, so I'm not sure whether we should drop this dependency altogether instead.