patch 8.0.0471: exit callback test sometimes fails

Problem:    Exit callback test sometimes fails.
Solution:   Add it to the list of flaky tests.

src/testdir/runtest.vim

@@ -167,6 +167,7 @@ let s:flaky = [
       \ 'Test_close_and_exit_cb()',
       \ 'Test_collapse_buffers()',
       \ 'Test_communicate()',
+      \ 'Test_exit_callback_interval()',
       \ 'Test_nb_basic()',
       \ 'Test_oneshot()',
       \ 'Test_pipe_through_sort_all()',

src/version.c

@@ -764,6 +764,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    471,
 /**/
     470,
 /**/