pull in base/stringprintf.h directly

Newer libbase has split this out of string_util.h, so we need to
explicitly include it now.

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

Change-Id: I73f83ba986d3d171f7e24c944f714d013614d25d
Reviewed-on: https://gerrit.chromium.org/gerrit/20200
Reviewed-by: Ryo Hashimoto <hashimoto@chromium.org>
Commit-Ready: Mike Frysinger <vapier@chromium.org>
Tested-by: Mike Frysinger <vapier@chromium.org>

chromeos_network.cc

diff --git a/chromeos_network.cc b/chromeos_network.cc
index 864d64e..4e2b23d 100644
--- a/chromeos_network.cc
+++ b/chromeos_network.cc
@@ -14,6 +14,7 @@
 #include "base/memory/scoped_ptr.h"
 #include "base/memory/scoped_vector.h"
 #include "base/string_util.h"
+#include "base/stringprintf.h"
 #include "chromeos/dbus/dbus.h"  // NOLINT
 #include "chromeos/dbus/service_constants.h"  // NOLINT
 #include "chromeos/glib/object.h"  // NOLINT