IPSec: fix compilation when building without libgcrypt

Change-Id: Iea3de2f0a54f9c0703afbeb42722d18da99f5c9a
Reviewed-on: https://code.wireshark.org/review/4090
Reviewed-by: Martin Mathieson <martin.r.mathieson@googlemail.com>
Petri-Dish: Pascal Quantin <pascal.quantin@gmail.com>
Tested-by: Petri Dish Buildbot <buildbot-no-reply@wireshark.org>
Reviewed-by: Evan Huus <eapache@gmail.com>