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>

package/gnuradio/gnuradio.mk

@@ -26,7 +26,8 @@ GNURADIO_CONF_OPTS = \
 	-DENABLE_DEFAULT=OFF \
 	-DENABLE_VOLK=ON \
 	-DENABLE_GNURADIO_RUNTIME=ON \
-	-DENABLE_GR_QTGUI=OFF
+	-DENABLE_GR_QTGUI=OFF \
+	-DXMLTO_EXECUTABLE=NOTFOUND
 
 # For third-party blocks, the gnuradio libraries are mandatory at
 # compile time.