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

tools/gn/action_target_generator.h

 // 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.
 
 #ifndef TOOLS_GN_ACTION_TARGET_GENERATOR_H_
 #define TOOLS_GN_ACTION_TARGET_GENERATOR_H_
 
 #include "base/macros.h"
+#include "tools/gn/target.h"
 #include "tools/gn/target_generator.h"
 
 // Populates a Target with the values from an action[_foreach] rule.
 class ActionTargetGenerator : public TargetGenerator {
  public:
   ActionTargetGenerator(Target* target,
                         Scope* scope,
                         const FunctionCallNode* function_call,
                         Target::OutputType type,
                         Err* err);
   ~ActionTargetGenerator() override;
 
  protected:
   void DoRun() override;
 
  private:
   bool FillScript();
   bool FillScriptArgs();
   bool FillDepfile();
 
   // Checks for errors in the outputs variable.
   bool CheckOutputs();
 
   Target::OutputType output_type_;
 
   DISALLOW_COPY_AND_ASSIGN(ActionTargetGenerator);
 };
 
 #endif  // TOOLS_GN_ACTION_TARGET_GENERATOR_H_