Silence deprecation warnings for PopplerAttachment->ctime, ->mtime
GTime is not Y2038-safe, and is now marked as deprecated. Don't trigger deprecation warnings for projects that include poppler headers but do not otherwise use GTime.
Part of #765 (closed).
If reviewers would prefer it, an alternative way to achieve the same result in this case would be to replace both uses of GTime
with gint32
.