patch 9.0.1611: v:maxcol can be changed in a :for loop

Problem:    v:maxcol can be changed in a :for loop.
Solution:   Check for read-only loop variable. (closes #12470)
diff --git a/src/version.c b/src/version.c
index a8f2104..2e3c240 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1611,
+/**/
     1610,
 /**/
     1609,