[pkg] Use the new deprecated_system_package alias

PKG-46

Change-Id: I678aaf4463e71c4942845ecb29b228d37cf1b4d7
diff --git a/BUILD.gn b/BUILD.gn
index 3717ede..f9c4714 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -30,7 +30,7 @@
 import("//build/package.gni")
 
 package("iperf") {
-  system_image = true
+  deprecated_system_image = true
 
   deps = [
     ":iperf3",