Code consuming the "crypto/tlscreds*.h" APIs doesn't need
to access its internals. Move the structure definitions to
the "tlscredspriv.h" private header (only accessible by
implementations). The public headers (in include/) still
forward-declare the structures typedef.

Note, tlscreds.c and 3 of the 5 modified source files already
include "tlscredspriv.h", so only add it to tls-cipher-suites.c
and tlssession.c.

Removing the internals from the public header solves a bug
introduced by commit 7de2e856 ("yank: Unregister function
when using TLS migration") which made migration/qemu-file-channel.c
include "io/channel-tls.h", itself sometime depends on GNUTLS,
leading to a build failure on OSX:

  [2/35] Compiling C object libmigration.fa.p/migration_qemu-file-channel.c.o
  FAILED: libmigration.fa.p/migration_qemu-file-channel.c.o
  cc -Ilibmigration.fa.p -I. -I.. -Iqapi [ ... ] -o libmigration.fa.p/migration_qemu-file-channel.c.o -c ../migration/qemu-file-channel.c
  In file included from ../migration/qemu-file-channel.c:29:
  In file included from include/io/channel-tls.h:26:
  In file included from include/crypto/tlssession.h:24:
  include/crypto/tlscreds.h:28:10: fatal error: 'gnutls/gnutls.h' file not found
  #include <gnutls/gnutls.h>
           ^~~~~~~~~~~~~~~~~
  1 error generated.

Reported-by: Stefan Weil <sw@weilnetz.de>
Suggested-by: Daniel P. Berrangé <berrange@redhat.com>
Resolves: https://gitlab.com/qemu-project/qemu/-/issues/407


Fixes: 7de2e856 ("yank: Unregister function when using TLS migration")
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Signed-off-by: Daniel P. Berrangé <berrange@redhat.com>