[pkg] Use the new deprecated_system_package alias

PKG-46

Change-Id: If21dfb4166503a515950cd2bbbb9a5cbdd09b3eb
diff --git a/BUILD.gn b/BUILD.gn
index 6870c8b..c4b6135 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -6,7 +6,7 @@
 
 package("leveldb_tests") {
   testonly = true
-  system_image = true
+  deprecated_system_image = true
 
   deps = [
     ":leveldb_tests_bin",