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)

src/version.c

@@ -757,6 +757,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    3509,
 /**/
     3508,
 /**/