tools/gn: Fix errors found by Facebook's flint tool.

tools/gn/exec_process.h

 // 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.
 
+#ifndef TOOLS_GN_EXEC_PROCESS_H_
+#define TOOLS_GN_EXEC_PROCESS_H_
+
 #include <string>
 
 namespace base {
 class CommandLine;
 class FilePath;
 }
 
 namespace internal {
+
 bool ExecProcess(const base::CommandLine& cmdline,
                  const base::FilePath& startup_dir,
                  std::string* std_out,
                  std::string* std_err,
                  int* exit_code);
+
 }  // namespace internal
+
+#endif  // TOOLS_GN_EXEC_PROCESS_H_