Build against the new JSON-C library name.

If json-c is built with the --disable-oldname-compat then bti will not
build.

This fixes it. Obviously it could be more complex but most distros use
the new name now as it's needed for both PHP and PulseAudio.

Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>