[build] Add system_image property to package()

Change-Id: Ic15ea6edbf5808fbfc313c89842caba0242e13b9
diff --git a/BUILD.gn b/BUILD.gn
index 67ccc5c..db9dd4a 100644
--- a/BUILD.gn
+++ b/BUILD.gn
@@ -59,6 +59,7 @@
 
 package("libc-tests") {
   testonly = true
+  system_image = true
 
   deps = [
     ":libc-large-tests",