manpage: Declare y2038 compliance (or lack thereof)

debian/bgpdump.txt

@@ -93,6 +93,9 @@ withdrawn prefixes, all those above that number are ignored.
 
 Files with a name longer than 999 characters cannot be processed.
 
+The bgpdump program is not y2038 compliant. This is inherited from
+the file format.
+
 
 DEBIAN
 ------