Fix fft app test for out-of-tree builds
Fix fft app test for out-of-tree builds

Former-commit-id: 41828e9b2deb9340b302da78e73662be0642607c
darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide