patch 8.2.4682: Vim9: can use :unlockvar for const variable

Problem:    Vim9: can use :unlockvar for const variable. (Ernie Rael)
Solution:   Check whether the variable is a const.
diff --git a/src/version.c b/src/version.c
index 79a3bad..8b7450c 100644
--- a/src/version.c
+++ b/src/version.c
@@ -751,6 +751,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    4682,
+/**/
     4681,
 /**/
     4680,