Update code to include base/files/file_util.h

file_util.h was moved from base to base/files
(https://codereview.chromium.org/468253002). This CL updates platform2
code to include base/files/file_util.h instead of base/file_util.h.

BUG=chromium:411001
TEST=Trybot run on paladin, release, and chromiumos-sdk builders.

Change-Id: I488925b54615e131e508a460dc1a27f88168f936
Reviewed-on: https://chromium-review.googlesource.com/216851
Reviewed-by: Alex Vakulenko <avakulenko@chromium.org>
Tested-by: Ben Chan <benchan@chromium.org>
Commit-Queue: Ben Chan <benchan@chromium.org>
diff --git a/dhcp_provider.cc b/dhcp_provider.cc
index decedec..4106de0 100644
--- a/dhcp_provider.cc
+++ b/dhcp_provider.cc
@@ -4,7 +4,7 @@
 
 #include "shill/dhcp_provider.h"
 
-#include <base/file_util.h>
+#include <base/files/file_util.h>
 #include <base/strings/stringprintf.h>
 
 #include "shill/control_interface.h"