add fftw to imagemagick build

added explicitly because despite it being a default, fft doesn't seem to work.

```
~/src/fft-test
❯ magick clown_orig.jpg -fft \
           \( +clone  -write clown_phase.png +delete \) +delete \
           -write clown_magnitude.png  -colorspace gray \
           -auto-level -evaluate log 100000  clown_spectrum.png
magick: delegate library support not built-in `clown_orig.jpg' (FFTW) @ warning/fourier.c/ForwardFourierTransformImage/914.
magick: unable to open image ' ': No such file or directory @ error/blob.c/OpenBlob/3527.
magick: no decode delegate for this image format `' @ error/constitute.c/ReadImage/572.

~/src/fft-test
❯ magick -version
Version: ImageMagick 7.1.0-15 Q16-HDRI aarch64 2021-11-20 https://imagemagick.org
Copyright: (C) 1999-2021 ImageMagick Studio LLC
License: https://imagemagick.org/script/license.php
Features: Cipher DPC HDRI
Delegates (built-in): bzlib cairo fontconfig freetype heic jng jp2 jpeg jxl lcms lzma pangocairo png rsvg tiff webp x xml zlib
Compiler: gcc (4.2)
```
As you can see, fft doesn't work, and the fftw delegate isn't listed as a built-in.
Unsure if this would work, as I am unable to configure a termux build environment (my pc is >10 years old, not powerful enough for android build environments; the build scripts don't seem to want to work on my tablet)

packages/imagemagick/build.sh

@@ -17,6 +17,7 @@ TERMUX_PKG_EXTRA_CONFIGURE_ARGS="
 --with-rsvg=yes
 --with-lzma
 --with-jxl=yes
+--with-fftw
 --disable-openmp
 ac_cv_func_ftime=no
 "