[software] Fix compilation warning for folded scheduled fft #1083

Job	Run time
check-opcodes	5s
tc-gcc	2m 11s
tc-llvm	4m 35s
riscv-isa-sim	12s
verilator	15s
verilator-model (minpool)	11s
verilator-model (mempool)	17s
unit-test	1m 48s
build-apps-omp (minpool)	49s
build-apps-omp (mempool)	41s
build-apps-gcc (minpool)	1m 20s
build-apps-gcc (mempool)	1m 28s
check-bootrom	38s
run-apps-gcc (minpool)	35s
run-apps-gcc (mempool)	4m 41s
tc-halide	22s
build-apps-llvm (minpool)	1m 28s
build-apps-llvm (mempool)	1m 33s
build-apps-halide (minpool)	1m 35s
build-apps-halide (mempool)	1m 32s
run-apps-llvm	0s
run-apps-halide (minpool)	29s
run-apps-halide (mempool)	3m 35s
clean-up	2s
clean-up-compile-runs (minpool)	2s
clean-up-compile-runs (mempool)	3s
	30m 27s

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