base|krb5: struct krb5_config_binding is public

base|krb5: struct krb5_config_binding is public

ea90ca86664c73fb8d415f3cc7baacdf8a6dd685
("Move some infra bits of lib/krb5/ to lib/base/ (2)") introduced
struct heim_config_binding to heimbase.h and removed the
struct krb5_config_binding definition from krb5.h.  It changed
the krb5_config_binding typedef to be based upon the heim_config_binding
typedef.

These changes broke out of tree callers of krb5_config_get_list()
and krb5_config_vget_list().  The internals of struct krb5_config_binding
are required by callers of krb5_config_get_list() and krb5_config_vget_list()
and the names must remain the same.

This change restores struct krb5_config_binding to krb5.h.  The
structure cannot be changed because it is public and leaves struct
heim_config_binding as an independent structure definition within
heimbase.h.  As a result struct heim_config_binding in heimbase.h must
remain binary compatible until such time as krb5_config_get_list() and
krb5_config_vget_list() are no longer supported.

Change-Id: I69b4fda3f656cc8daa8f5fcd0c7151cee222fc8c