pull in base/stringprintf.h explicitly

Newer libbase no longer implicitly includes the stringprintf.h header,
so building against it fails with missing StringPrintf definitions.

BUG=chromium-os:25872
TEST=`emerge-x86-alex update_engine` works with old & new libbase

Change-Id: Ia1ea842449c0382ee4c8e5abd3ddfb3012c3ff2f
Reviewed-on: https://gerrit.chromium.org/gerrit/19774
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>
diff --git a/utils.cc b/utils.cc
index 1c57fbf..3232061 100644
--- a/utils.cc
+++ b/utils.cc
@@ -1,4 +1,4 @@
-// Copyright (c) 2011 The Chromium OS Authors. All rights reserved.
+// Copyright (c) 2012 The Chromium OS Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
@@ -22,9 +22,10 @@
 #include <base/eintr_wrapper.h>
 #include <base/file_path.h>
 #include <base/file_util.h>
+#include <base/logging.h>
 #include <base/rand_util.h>
 #include <base/string_util.h>
-#include <base/logging.h>
+#include <base/stringprintf.h>
 #include <google/protobuf/stubs/common.h>
 #include <rootdev/rootdev.h>