Fix/hide ifft to factory

[vitalsong]

No description provided.

[codecov]

Codecov Report

Attention: Patch coverage is 92.52336% with 8 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
lib/fft/cmplx-ifft.h	84.61%	2 Missing ⚠️
lib/fft/ifft.cpp	66.66%	2 Missing ⚠️
lib/fft/real-fft.cpp	93.54%	2 Missing ⚠️
lib/fft/real-ifft.cpp	94.59%	2 Missing ⚠️

Files with missing lines	Coverage Δ
include/dsplib/ifft.h	100.00% <100.00%> (ø)
lib/fft/factory.cpp	100.00% <100.00%> (ø)
lib/fft/fft.cpp	83.33% <ø> (ø)
lib/fft/cmplx-ifft.h	84.61% <84.61%> (ø)
lib/fft/ifft.cpp	81.81% <66.66%> (-11.74%)	⬇️
lib/fft/real-fft.cpp	93.54% <93.54%> (ø)
lib/fft/real-ifft.cpp	94.59% <94.59%> (ø)

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