apps/fft: ensure that "input" has a well-defined name
apps/fft: ensure that "input" has a well-defined name

Otherwise, build errors can result on systems without name inference


Former-commit-id: f39643d25e73d0106c151d04cc583c864ee4f3ac
Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,darkbuck/Halide,Trass3r/Halide