Inflict PDF_ENABLE_XFA ifdefs on XFA fpdfsdk/

R=thestig@chromium.org

Review URL: https://codereview.chromium.org/1477583002 .
diff --git a/fpdfsdk/src/formfiller/FFL_FormFiller.cpp b/fpdfsdk/src/formfiller/FFL_FormFiller.cpp
index 9817f05..867bda0 100644
--- a/fpdfsdk/src/formfiller/FFL_FormFiller.cpp
+++ b/fpdfsdk/src/formfiller/FFL_FormFiller.cpp
@@ -562,10 +562,12 @@
 
 void CFFL_FormFiller::SaveData(CPDFSDK_PageView* pPageView) {}
 
+#ifdef PDF_ENABLE_XFA
 FX_BOOL CFFL_FormFiller::IsFieldFull(CPDFSDK_PageView* pPageView) {
   return FALSE;
 }
 
+#endif
 void CFFL_FormFiller::SetChangeMark() {
   m_pApp->FFI_OnChange();
 }