[build] Add system_image property to package()

Change-Id: I54cba3a6b61eb15598a0bb80160f9ddc895ab2dc
diff --git a/BUILD.gn b/BUILD.gn
index d659002..c02d3e7 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -6,6 +6,8 @@
 
 package("leveldb_tests") {
   testonly = true
+  system_image = true
+
   deps = [
     ":leveldb_tests_bin",
   ]