package/gqrx: remove unneeded fftw dependency

package/gqrx: remove unneeded fftw dependency

Thanks to the new fftw package organization, gnuradio already selects
the appropriate fftw precision, and there is no need to propagate the
"depends on BR2_PACKAGE_FFTW_PRECISION_SINGLE" that used to be present
in the gnuradio package.

Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@trabucayre.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Reviewed-by: "Yann E. MORIN" <yann.morin.1998@free.fr>
[Thomas: reorder "depends on" as suggested by Yann]
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>