graphics/vapoursynth-fmtconv: unbreak on non-x86

In file included from ../../src/fmtc/Bitdepth.cpp:28:
In file included from ./../../src/fmtc/Bitdepth.h:30:
In file included from ./../../src/conc/ObjPool.h:44:
In file included from ./../../src/conc/CellPool.h:31:
./../../src/conc/AtomicInt.h:131:20: error: call to non-static member function without an object argument

std::atomic <T>::is_lock_free,
~~~~~~~~~~~~~~~~~^~~~~~~~~~~~

./../../src/conc/CellPool.h:95:18: note: in instantiation of template class 'conc::AtomicInt<unsigned long>' requested here

CountCells     _nbr_avail_cells;
               ^

Obtained from: upstream