patch 8.2.1515: Vim9: can create s:var in legacy script but cannot unlet

Problem:    Vim9: can create s:var in legacy script but cannot unlet.
Solution:   Allow :unlet for legacy script var.
diff --git a/src/version.c b/src/version.c
index 02457eb..65b9d0f 100644
--- a/src/version.c
+++ b/src/version.c
@@ -755,6 +755,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1515,
+/**/
     1514,
 /**/
     1513,