GN: Replace mojo/system.gni with forwarding groups

mojo/examples/wget/BUILD.gn

 # Copyright 2014 The Chromium Authors. All rights reserved.
 # 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_examples.gypi:mojo_wget
 shared_library("wget") {
   output_name = "mojo_wget"
 
   deps = [
+    "//mojo/public/c/system:for_shared_library",
     "//mojo/public/cpp/application:standalone",
     "//mojo/public/cpp/bindings",
     "//mojo/public/cpp/utility",
     "//mojo/services/public/interfaces/network",
   ]
-  deps += mojo_system_for_shared_library
 
   sources = [
     "wget.cc"
   ]
 }