patch 8.2.1791: Vim9: debugger test fails

Problem:    Vim9: debugger test fails.
Solution:   Use "var" instead of "let".

src/testdir/test_debugger.vim

@@ -859,7 +859,7 @@ func Test_Backtrace_DefFunction()
     vim9script
 
     def DoAThing(): number
-      let a = 100 * 2
+      var a = 100 * 2
       a += 3
       return a
     enddef

src/version.c

@@ -750,6 +750,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    1791,
 /**/
     1790,
 /**/