kdc: correctly generate PAC TGS signature

kdc: correctly generate PAC TGS signature

When generating an AS-REQ, the TGS signature was incorrectly generated using
the server key, which would fail to validate if the server was not also the
TGS. Fix this.

Patch from Isaac Bourkis <iboukris@gmail.com>.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=14642

[jsutton@samba.org Backported from Heimdal commit
 e7863e2af922809dad25a2e948e98c408944d551
 - Samba's Heimdal version does not have the generate_pac() helper
 function.
 - Samba's Heimdal version does not use the 'r' context variable.
]

Signed-off-by: Joseph Sutton <josephsutton@catalyst.net.nz>
Reviewed-by: Andrew Bartlett <abartlet@samba.org>