patch 8.1.1539: not easy to define a variable and lock it

Problem:    Not easy to define a variable and lock it.
Solution:   Add ":const".
diff --git a/src/version.c b/src/version.c
index 4a6fb00..45af9c0 100644
--- a/src/version.c
+++ b/src/version.c
@@ -778,6 +778,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1539,
+/**/
     1538,
 /**/
     1537,