XFA build fix.
XFA build fix.

We need to pull in the fgas_base.h file before looking at the _FX_OS_ checks
otherwise they become true and windows XFA build fails.

TBR=tsepez@chromium.org
BUG=pdfium:65

Review URL: https://codereview.chromium.org/1725843002 .

DrAlexx/pdfium,DrAlexx/pdfium,DrAlexx/pdfium,DrAlexx/pdfium