Do IWYU for cpdf_dictionary.h.

Move some method implementations out of headers in the process.

Change-Id: I8701e360d4addd9aec39fe887d932209985d2443
Reviewed-on: https://pdfium-review.googlesource.com/c/43608
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/fpdfsdk/cpdfsdk_interform.cpp b/fpdfsdk/cpdfsdk_interform.cpp
index d8f999e..b7b7109 100644
--- a/fpdfsdk/cpdfsdk_interform.cpp
+++ b/fpdfsdk/cpdfsdk_interform.cpp
@@ -16,6 +16,7 @@
 #include "core/fpdfapi/page/cpdf_page.h"
 #include "core/fpdfapi/parser/cfdf_document.h"
 #include "core/fpdfapi/parser/cpdf_array.h"
+#include "core/fpdfapi/parser/cpdf_dictionary.h"
 #include "core/fpdfapi/parser/cpdf_document.h"
 #include "core/fpdfapi/parser/cpdf_stream.h"
 #include "core/fpdfdoc/cpdf_actionfields.h"