libgcrypt: fix CVE-2017-9526

In libgcrypt before 1.7.7, an attacker who learns the EdDSA session key (from
side-channel observation during the signing process) can easily recover the
long-term secret key. 1.7.7 makes a cipher/ecc-eddsa.c change to store this
session key in secure memory, to ensure that constant-time point operations are
used in the MPI library.

Signed-off-by: Ross Burton <ross.burton@intel.com>
Signed-off-by: Richard Purdie <richard.purdie@linuxfoundation.org>