Rename interform files to interactiveform.

Just rename the files in this CL.

Change-Id: I39f7fefa885977fffab6c909762b05484a1c923d
Reviewed-on: https://pdfium-review.googlesource.com/c/43814
Reviewed-by: Tom Sepez <tsepez@chromium.org>
Commit-Queue: Lei Zhang <thestig@chromium.org>
diff --git a/fpdfsdk/cpdfsdk_helpers.cpp b/fpdfsdk/cpdfsdk_helpers.cpp
index dc8dc80..6a0cf85 100644
--- a/fpdfsdk/cpdfsdk_helpers.cpp
+++ b/fpdfsdk/cpdfsdk_helpers.cpp
@@ -14,7 +14,7 @@
 #include "core/fpdfapi/parser/cpdf_document.h"
 #include "core/fpdfapi/parser/cpdf_stream_acc.h"
 #include "core/fpdfdoc/cpdf_annot.h"
-#include "core/fpdfdoc/cpdf_interform.h"
+#include "core/fpdfdoc/cpdf_interactiveform.h"
 #include "core/fpdfdoc/cpdf_metadata.h"
 #include "public/fpdf_ext.h"