make AT_FFTW_ENABLED available to fb internal
Summary: follow up on D25375320 (https://github.com/pytorch/pytorch/commit/b89c32849352bf2fbb8f49749aa2fb0305a38c96).
Test Plan: buck build
Reviewed By: samestep
Differential Revision: D25410973
fbshipit-source-id: 6c2627951a98d270d341b33538431644d03bed16
