patch 8.0.1595: no autocommand triggered before exiting

Problem:    No autocommand triggered before exiting.
Solution:   Add the ExitPre autocommand event.
diff --git a/src/vim.h b/src/vim.h
index 563d328..3b458a0 100644
--- a/src/vim.h
+++ b/src/vim.h
@@ -1277,6 +1277,7 @@
     EVENT_COLORSCHEME,		/* after loading a colorscheme */
     EVENT_COMPLETEDONE,		/* after finishing insert complete */
     EVENT_DIRCHANGED,		/* after changing directory as a result of user cmd */
+    EVENT_EXITPRE,		/* before exiting */
     EVENT_FILEAPPENDPOST,	/* after appending to a file */
     EVENT_FILEAPPENDPRE,	/* before appending to a file */
     EVENT_FILEAPPENDCMD,	/* append to a file using command */