Restored ngx_event_aio_t layout for debug logging.

The "fd" field should be after 3 pointers for ngx_event_ident() to use it.
This was broken by ccad84a174e0.  While it does not seem to be currently used
for aio-related events, it should be a good idea to preserve the correct
layout nevertheless.

src/event/ngx_event.h

@@ -152,12 +152,12 @@ struct ngx_event_aio_s {
     ngx_event_handler_pt       handler;
     ngx_file_t                *file;
 
+    ngx_fd_t                   fd;
+
 #if (NGX_HAVE_AIO_SENDFILE || NGX_COMPAT)
     ssize_t                  (*preload_handler)(ngx_buf_t *file);
 #endif
 
-    ngx_fd_t                   fd;
-
 #if (NGX_HAVE_EVENTFD)
     int64_t                    res;
 #endif