[GX] Fix build with some compilers Eg. https://ci.appveyor.com/project/behdad/harfbuzz/build/1.0.48/job/o9mnd33kcdeeg30r hb-open-type-private.hh:103:29: error: static data member 'OT::Device::<anonymous union>::<anonymous struct>::static_size' in unnamed class [-fpermissive]