Do IWYU in testing/utils/file_util.cpp.

g++ complained about memcpy().

Change-Id: Ia4871e06255a36fc90fb9eca6fdf97d6331725da
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/64410
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>

testing/utils/file_util.cpp

diff --git a/testing/utils/file_util.cpp b/testing/utils/file_util.cpp
index ffd387b..5d26d98 100644
--- a/testing/utils/file_util.cpp
+++ b/testing/utils/file_util.cpp
@@ -5,6 +5,7 @@
 #include "testing/utils/file_util.h"
 
 #include <stdio.h>
+#include <string.h>
 
 #include "testing/utils/path_service.h"