patch 9.0.0840: cannot change a slice of a const list

Problem:    Cannot change a slice of a const list. (Takumi KAGIYAMA)
Solution:   Remove the const flag from the slice type. (closes #11490)
diff --git a/src/version.c b/src/version.c
index c454557..e7dc5a5 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    840,
+/**/
     839,
 /**/
     838,