Fix GN PDFium build when building all.

The problem was FT_INTERNAL_DEBUG_H wasn't being defined because FT2_BUILD_LIBRARY wasn't set. So even if PDFium isn't using bundled FreeType, the FreeType target needs FT2_BUILD_LIBRARY defined. The GYP build was already correct.

BUG=453844
TBR=brettw

Review URL: https://codereview.chromium.org/891993003