GN: Replace mojo/system.gni with forwarding groups

mojo/examples/wget/BUILD.gn

Index: mojo/examples/wget/BUILD.gn
diff --git a/mojo/examples/wget/BUILD.gn b/mojo/examples/wget/BUILD.gn
index 70e4ea087e54d1fc8f092bb876ae6797137bd791..448e4ec20e24d98a3b9ab0424c6bf6a97477af66 100644
--- a/mojo/examples/wget/BUILD.gn
+++ b/mojo/examples/wget/BUILD.gn
@@ -2,19 +2,17 @@
 # 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"