[pkg] Use the new deprecated_system_package alias

PKG-46

Change-Id: I6fa54fe387836c72152ac15482eee8dbc4eae7c0
diff --git a/BUILD.gn b/BUILD.gn
index 1bb105f..3a0f883 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -347,7 +347,7 @@
 
 package("sbase") {
   testonly = true
-  system_image = true
+  deprecated_system_image = true
 
   deps = [
     ":all",