package/liquid-dsp: add missing dependency on fftw

package/liquid-dsp: add missing dependency on fftw

When one of BR2_PACKAGE_FFTW_PRECISION_* is enabled, liquid-dsp links
against fftw3f, fftw3 or fftw3l, but forgets to add the fftw package
in its dependencies. It works fine in practice because "fftw" is
before "liquid-dsp" in the alphabetic ordering, but building with
"make liquid-dsp" or with per-package directory causes a build
failure.

Fix that by adding the missing dependencies.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Tested-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Reviewed-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>