Fix IWYU in cpdf_stringarchivestream.h

The header refers to a std:: type on old line 12.

Change-Id: Ia41409919ceca4a517152f0e07c089e2a442fe60
Reviewed-on: https://pdfium-review.googlesource.com/c/pdfium/+/83150
Auto-Submit: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Lei Zhang <thestig@chromium.org>
diff --git a/core/fpdfapi/edit/cpdf_stringarchivestream.h b/core/fpdfapi/edit/cpdf_stringarchivestream.h
index b1b5f72..4107122 100644
--- a/core/fpdfapi/edit/cpdf_stringarchivestream.h
+++ b/core/fpdfapi/edit/cpdf_stringarchivestream.h
@@ -5,6 +5,8 @@
 #ifndef CORE_FPDFAPI_EDIT_CPDF_STRINGARCHIVESTREAM_H_
 #define CORE_FPDFAPI_EDIT_CPDF_STRINGARCHIVESTREAM_H_
 
+#include <iosfwd>
+
 #include "core/fxcrt/fx_stream.h"
 
 class CPDF_StringArchiveStream final : public IFX_ArchiveStream {