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/real_system_state.cc b/real_system_state.cc
index f093aad..faac5cd 100644
--- a/real_system_state.cc
+++ b/real_system_state.cc
@@ -4,7 +4,7 @@
 
 #include "update_engine/real_system_state.h"
 
-#include <base/file_util.h>
+#include <base/files/file_util.h>
 #include <base/time/time.h>
 
 #include "update_engine/constants.h"