Fix STL IWYU lint errors.

Also use std::wstring in one place where appropriate.

Change-Id: Idfc21a22af9619c927c1413fcff22a05da370c65
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/81474
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Reviewed-by: Tom Sepez <tsepez@chromium.org>
diff --git a/fpdfsdk/fpdf_signature_embeddertest.cpp b/fpdfsdk/fpdf_signature_embeddertest.cpp
index e91d9c2..db31990 100644
--- a/fpdfsdk/fpdf_signature_embeddertest.cpp
+++ b/fpdfsdk/fpdf_signature_embeddertest.cpp
@@ -2,6 +2,8 @@
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
+#include <vector>
+
 #include "public/fpdf_signature.h"
 #include "testing/embedder_test.h"
 #include "testing/fx_string_testhelpers.h"