libubox: correct json-c handling

Fixes http://autobuild.buildroot.net/results/5a8/5a85e0e132f38a4a1a42c2b041f66003b64e748a/


(and a bunch of similar failures)

libubox has optional json-c handling, including the legacy version which
used libjson.so.  Unfortunately this support is buggy, so we end up with the
wrong compiler flags, and more importantly it gets confused if libjson is
enabled, as that also provides a (incompatible) libjson.so.

Fix it by disabling the legacy handling.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>