Revert wrong fix for ECDSA.

* src/certreq.c (ksba_certreq_set_sig_val): Add back MSB handling of
0x80 for ECDSA.

This was removed because MSB handling was implemented in gpg-agent for
smartcard (historical reason). Since gpg-agent should return
consistent result for both of smartcard keys and normal disk keys,
gpg-agent has been fixed to libgcrypt format (with no prefixed 0).

• Fixes-commit: 5d9278f3d13050eddd68d8a1c490274a4f631f5f
• Signed-off-by: NIIBE Yutaka <gniibe@fsij.org>