patch 8.1.1540: cannot build without the +eval feature

Problem:    Cannot build without the +eval feature.
Solution:   Define ex_const if needed.

src/version.c

@@ -777,6 +777,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1540,
 /**/
     1539,
 /**/