gnuradio: needs host-python2

gnuradio: needs host-python2

gnuradio needs host-python2 however there is no way to enforce this so
add a dependency on !BR2_PACKAGE_PYTHON3.
Indeed, if BR2_PACKAGE_PYTHON3 is selected, then buildroot will only
build host-python3.
This issue was not raised in the previous version of gnuradio as
host-python-cheetah was the only dependency and host-python-cheetah
enforces python2 through HOST_PYTHON_CHEETAH_NEEDS_HOST_PYTHON however
we now have host-python-six and host-python-mako dependencies and it
does not seem right to enforces python2 on those packages

Fixes:
 - http://autobuild.buildroot.net/results/a502fef3e2a40f38ca63332440b0b6bd2ed11d5a

Signed-off-by: Fabrice Fontaine <fontaine.fabrice@gmail.com>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>