json: fix json_events_cb() declaration when libjansson is not present

json: fix json_events_cb() declaration when libjansson is not present

When nftables is configured without libjansson support, the following
compilation error is shown:

monitor.c: In function ‘netlink_echo_callback’:
monitor.c:910:10: error: too many arguments to function ‘json_events_cb’
   return json_events_cb(nlh, &echo_monh);
          ^~~~~~~~~~~~~~

This patch makes a declaration of the json_events_cb() function
consistent.

Fixes: bb32d8db9a12 ("JSON: Add support for echo option")
Signed-off-by: Laura Garcia Liebana <nevola@gmail.com>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>