patch 9.0.0831: compiler warning for redefining HAVE_DUP

Problem:    Compiler warning for redefining HAVE_DUP.
Solution:   Undefine HAVE_DUP if needed. (Ozaki Kiichi, closes #11484)
diff --git a/src/version.c b/src/version.c
index a914e78..ca70914 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    831,
+/**/
     830,
 /**/
     829,