patch 8.2.0602: :unlet $VAR does not work properly

Problem:    :unlet $VAR does not work properly.
Solution:   Make ":lockvar $VAR" fail.  Check the "skip" flag.
diff --git a/src/version.c b/src/version.c
index 21400bb..66f5365 100644
--- a/src/version.c
+++ b/src/version.c
@@ -747,6 +747,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    602,
+/**/
     601,
 /**/
     600,