[build] Add system_image property to package()

Change-Id: I0ca46724ef93da0d27873f49ea384b26c4bcaf05

BUILD.gn

diff --git a/BUILD.gn b/BUILD.gn
index f8f8b0a..5dc1d95 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -30,6 +30,7 @@
 
 package("asio_unittests") {
   testonly = true
+  system_image = true
 
   deps = [
     ":tests",