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.

Signed-off-by: Simon McVittie <smcv@debian.org>
diff --git a/glib/poppler-attachment.h b/glib/poppler-attachment.h
index 5a8a7e0..03b8395 100644
--- a/glib/poppler-attachment.h
+++ b/glib/poppler-attachment.h
@@ -54,6 +54,8 @@
 					       gpointer      data,
 					       GError      **error);
 
+/* GTime is deprecated, but is part of our ABI here (see #715, #765). */
+G_GNUC_BEGIN_IGNORE_DEPRECATIONS
 struct _PopplerAttachment
 {
   GObject parent;
@@ -65,6 +67,7 @@
   GTime ctime;
   GString *checksum;
 };
+G_GNUC_END_IGNORE_DEPRECATIONS
 
 typedef struct _PopplerAttachmentClass
 {