bind: disable libjson support

It conflicts with jsoncpp, bind probes for json/json.h first, but that
header is installed by jsoncpp, which is completely different from
json-c.
Since it's not clear who's correct here (there might be some other
json-c predecessor/version that installs there as well) and the same
functionality (stats channel) is provided by libxml2 as well, just
disable libjson support completely.

Fixes:
http://autobuild.buildroot.net/results/226/2262c9b46663ea7a45e128a5fd7ff30417c2c2a7/build-end.log


(indirectly, it was probing aboslute directories while searching for it)

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>