patch 8.2.3509: undo file is not synced

Problem:    Undo file is not synced. (Sami Farin)
Solution:   Sync the undo file if 'fsync' is set. (Christian Brabandt,
            closes #8879, closes #8920)
diff --git a/src/version.c b/src/version.c
index 2125972..31f0015 100644
--- a/src/version.c
+++ b/src/version.c
@@ -758,6 +758,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    3509,
+/**/
     3508,
 /**/
     3507,