lib/gpg: Correct missing line prefix with bad signatures
lib/gpg: Correct missing line prefix with bad signatures

In the case the signature time was bad, a line prefix was missing from the
result of `ostree_gpg_verify_result_describe_variant()`.

Closes: #1092
Approved by: cgwalters

GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree,GNOME/ostree