passdb: Increase ABI version to 0.28.0

The change from c906153c lib: Remove some unused code
removed functions, but only updated the minor version
of the ABI. Update the passdb version to 0.28.0
to reflect this change.
file.

Signed-off-by: Christof Schmitt <cs@samba.org>
Reviewed-by: Jeremy Allison <jra@samba.org>

Autobuild-User(master): Jeremy Allison <jra@samba.org>
Autobuild-Date(master): Thu Mar  7 01:30:49 UTC 2019 on sn-devel-144