patch 8.1.1540: cannot build without the +eval feature

Problem:    Cannot build without the +eval feature.
Solution:   Define ex_const if needed.
diff --git a/src/version.c b/src/version.c
index 45af9c0..7e9cac3 100644
--- a/src/version.c
+++ b/src/version.c
@@ -778,6 +778,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1540,
+/**/
     1539,
 /**/
     1538,