package/gnuradio: disable xml document generation
Fixes
http://autobuild.buildroot.net/results/f94/f941d84c781b524530770f5b9360863a821e8ba1/
Signed-off-by: Matthew Weber <matthew.weber@rockwellcollins.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>