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

tools/gn/tutorial/hello.cc

Index: tools/gn/tutorial/hello.cc
diff --git a/tools/gn/tutorial/hello.cc b/tools/gn/tutorial/hello.cc
index 6e7850425b8c4488e699dcf553be96fd6f605460..8b25cd9db8218e61ec4197d2859bcb2d2b66d794 100644
--- a/tools/gn/tutorial/hello.cc
+++ b/tools/gn/tutorial/hello.cc
@@ -2,10 +2,10 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
-#include <stdio.h>
-
 #include "tools/gn/tutorial/hello.h"
 
+#include <stdio.h>
+
 void Hello(const char* who) {
   printf("Hello, %s.\n", who);
 }