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

tools/gn/exec_process.h

Index: tools/gn/exec_process.h
diff --git a/tools/gn/exec_process.h b/tools/gn/exec_process.h
index 74622614819817c93f8003cb6c2c72a67641b5f8..4d010d519f9016c7210104284f132e3b429e6a99 100644
--- a/tools/gn/exec_process.h
+++ b/tools/gn/exec_process.h
@@ -2,6 +2,9 @@
 // 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 {
@@ -10,9 +13,13 @@ 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_