libksba: update to 1.8.0

libksba: update to 1.8.0

Minor version bump on the GnuPG X.509 library. Highlights from
upstream's NEWS:

 1.8.0 (2026-05-13)
  * New function ksba_cms_get_attribute.
  * Support building of unsigned attributes with
    ksba_cms_add_attribute.

 1.7.0 (2026-05-07)
  * Add support for building AuthEnvelopedData.
  * New function ksba_cms_add_attribute.
  * Fix silent truncation of 64 bit length fields.
  * Fix incorrect overflow guard condition in _ksba_ber_read_tl.

The added interfaces (ksba_cms_add_attribute, _get_attribute) are
required by GnuPG >= 2.5.20 to expose gpgsm's --attribute option.

Signed-off-by: Daniel Golle <redacted>