Bug#953789: unnecessary build-dep on libjson-c-dev

[dann frazier <dannf@dannf.org>]

Source: mstflint
Version: 4.13.3+2-1

mstflint build-deps on libjson-c-dev, but doesn't appear to use it.
It builds fine w/o it installed, and none of the produced binaries
dynamically link to libjson-c:

  $ dpkg-deb -x /var/cache/apt/archives/mstflint_4.13.3+2-1_amd64.deb x/
  $ find x -type f | xargs ldd | grep json
  $

We don't currently build any of the subcommands that #include json
headers (mlxarchive, mlxfwupdate, mlxlink, from my grep) as they are
not ./configure --enabled. And even if we did, they apparently
wouldn't build w/ the system library anyway. For example:
  https://github.com/Mellanox/mstflint/issues/305

 -dann