Re-add keyid to gpg signature dictionary
Re-add keyid to gpg signature dictionary

336b34cf851cd85c7a049d6155c0fdba2ad20698 changed the way
the keyid is parsed from gpg signatures and accidentally removed
the keyid from the signature dict. This commit re-adds it.

secure-systems-lab/securesystemslib,secure-systems-lab/securesystemslib