patch 8.2.3420: _REENTRANT defined more than once

Problem:    _REENTRANT defined more than once.
Solution:   Fix configure script. (Christian Brabandt, closes #8852)
diff --git a/src/version.c b/src/version.c
index 664f4ca..6d64ffe 100644
--- a/src/version.c
+++ b/src/version.c
@@ -756,6 +756,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    3420,
+/**/
     3419,
 /**/
     3418,