patch 9.0.1596: :registers command does not work in sandbox

Problem:    :registers command does not work in sandbox.
Solution:   Add flag to the command. (closes #12473)
diff --git a/src/version.c b/src/version.c
index 4b04193..132a0d9 100644
--- a/src/version.c
+++ b/src/version.c
@@ -696,6 +696,8 @@
 static int included_patches[] =
 {   /* Add new patch number below this line */
 /**/
+    1596,
+/**/
     1595,
 /**/
     1594,