mtr: fix JSON output format, add libjansson

Currently if you try to use `mtr` with `-j`/`--json` flag you get:
```
mtr: invalid option -- 'j'
```
In order to make this work it has to be built with `libjansson`:
https://github.com/traviscross/mtr/issues/358

Signed-off-by: Jakub Sokołowski <jakub@status.im>

pkgs/tools/networking/mtr/default.nix

@@ -1,5 +1,5 @@
 { stdenv, lib, fetchFromGitHub, autoreconfHook, pkg-config
-, libcap, ncurses
+, libcap, ncurses, jansson
 , withGtk ? false, gtk3 }:
 
 stdenv.mkDerivation rec {
@@ -28,7 +28,7 @@ stdenv.mkDerivation rec {
 
   nativeBuildInputs = [ autoreconfHook pkg-config ];
 
-  buildInputs = [ ncurses ]
+  buildInputs = [ ncurses jansson ]
     ++ lib.optional withGtk gtk3
     ++ lib.optional stdenv.isLinux libcap;