patch 8.1.1543: const test fails with small features

Problem:    Const test fails with small features.
Solution:   Don't unlet non-existing variables.
diff --git a/src/version.c b/src/version.c
index 931b3af..0b036d0 100644
--- a/src/version.c
+++ b/src/version.c
@@ -778,6 +778,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1543,
+/**/
     1542,
 /**/
     1541,