Warning fixes and clean up in FFT and MDCT fix-pt files

[x] Warning fixes in fft_fx.c, fft_cldfb_fx.c, decoder.c,
ivas_mdct_imdct_fx.c files.
[x] Few other related clean-ups like moving function declarations
from .c files to appropriate header file.
[x] Testing: Unit tests for FFT/IFFT and MDCT/IMDCT.