Fix IWYU for std::back_inserter().

Include <iterator> in pdfium_test_dump_helper.cc so future MSVC updates
won't fail to compile this file.

Change-Id: Icc8ef8294b86ca2f99b4d2ffedfaf9708fb4eebd
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/93250
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
diff --git a/samples/pdfium_test_dump_helper.cc b/samples/pdfium_test_dump_helper.cc
index 0be3f0d..221eb81 100644
--- a/samples/pdfium_test_dump_helper.cc
+++ b/samples/pdfium_test_dump_helper.cc
@@ -9,6 +9,7 @@
 
 #include <algorithm>
 #include <functional>
+#include <iterator>
 #include <string>
 #include <utility>