GN: Replace mojo/system.gni with forwarding groups

mojo/services/network/BUILD.gn

Index: mojo/services/network/BUILD.gn
diff --git a/mojo/services/network/BUILD.gn b/mojo/services/network/BUILD.gn
index cee4b4aac4a3ce6c47dcd097fa88c674c0c98a20..82dd07b25f9afbc835839a127a71802f3abd091d 100644
--- a/mojo/services/network/BUILD.gn
+++ b/mojo/services/network/BUILD.gn
@@ -2,17 +2,15 @@
 # Use of this source code is governed by a BSD-style license that can be
 # found in the LICENSE file.
 
-import("//mojo/system.gni")
-
 # GYP version: mojo/mojo_services.gypi:mojo_network_service
 shared_library("network") {
   output_name = "mojo_network_service"
 
   deps = [
     ":lib",
+    "//mojo/public/c/system:for_shared_library",
     "//mojo/services/public/interfaces/network",
   ]
-  deps += mojo_system_for_shared_library
 
   sources = [ "main.cc" ]
 }