Fix implicit declarations when HAVE_OPENSSL_ENGINE is unset

Fix implicit declarations when HAVE_OPENSSL_ENGINE is unset

This fixes a regression in commit 8e95e20913:
on amd64, an implicit declaration of ERR_error_string leads to segfaults
if the returned pointer does not fit in an 'int'.

Trac: #792

Signed-off-by: Julien Muchembled <jm@nexedi.com>
Acked-by: Steffan Karger <steffan.karger@fox-it.com>
Message-Id: <20161216163218.25449-1-jm@nexedi.com>
URL: https://www.mail-archive.com/openvpn-devel@lists.sourceforge.net/msg13591.html
Signed-off-by: Gert Doering <gert@greenie.muc.de>