Fix RFC 5587 const pointer typedefs

gss_const_ctx_id_t, gss_const_cred_id_t, and gss_const_name_t are
supposed to be const pointers to the appropriate structures, not the
structures themselves.  These are not used by any prototypes yet, and
no application would have any reason to use them as they are, so it
should be safe to change them within the public header.

ticket: 7567 (new)
target_version: 1.11.1
tags: pullup