tools/gn: Fix the includes of target_generator.h header.

tools/gn/group_target_generator.cc

 // Copyright (c) 2013 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.
 
 #include "tools/gn/group_target_generator.h"
 
+#include "tools/gn/target.h"
 #include "tools/gn/variables.h"
 
 GroupTargetGenerator::GroupTargetGenerator(
     Target* target,
     Scope* scope,
     const FunctionCallNode* function_call,
     Err* err)
     : TargetGenerator(target, scope, function_call, err) {
 }
 
 GroupTargetGenerator::~GroupTargetGenerator() {
 }
 
 void GroupTargetGenerator::DoRun() {
   target_->set_output_type(Target::GROUP);
   // Groups only have the default types filled in by the target generator
   // base class.
 
   // Before there was a deps/public_deps split, a group acted like all deps
   // are public. During a transition period, if public_deps is not defined,
   // treat all deps as public. This should be removed and existing groups
   // updated to use "public_deps" when needed.
   if (scope_->GetValue(variables::kDeps, false) &&
       !scope_->GetValue(variables::kPublicDeps, false)) {
     std::swap(target_->private_deps(), target_->public_deps());
   }
 }