[pkg] Use the new deprecated_system_package alias

PKG-46

Change-Id: I42f08349f4a1016bbfd27dcc0a80f5083de650ab
diff --git a/BUILD.gn b/BUILD.gn
index 571abe5..19eb0f3 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -30,7 +30,7 @@
 
 package("asio_unittests") {
   testonly = true
-  system_image = true
+  deprecated_system_image = true
 
   deps = [
     ":tests",