GUACAMOLE-861: Fixes WINDOWS_TIME calculation
GUACAMOLE-861: Fixes WINDOWS_TIME calculation

This fixes the UNIX time to FILETIME conversion in WINDOWS_TIME macro,
according to MSDN (addition instead of subtraction).

apache/guacamole-server,apache/guacamole-server,mike-jumper/incubator-guacamole-server,apache/guacamole-server,glyptodon/guacamole-server,glyptodon/guacamole-server,mike-jumper/incubator-guacamole-server,glyptodon/guacamole-server,mike-jumper/incubator-guacamole-server,mike-jumper/incubator-guacamole-server