patch 8.2.1791: Vim9: debugger test fails

Problem:    Vim9: debugger test fails.
Solution:   Use "var" instead of "let".
diff --git a/src/version.c b/src/version.c
index a9b6e5d..40a5ea8 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1791,
+/**/
     1790,
 /**/
     1789,