lib: Fix array size in audit_logging

lib: Fix array size in audit_logging

../lib/audit_logging/audit_logging.c: In function ‘json_add_timestamp’:
../lib/audit_logging/audit_logging.c:603:12: error: ‘%s’ directive
       output may be truncated writing up to 9 bytes into a region of size
       between 0 and 43 [-Werror=format-truncation=]
   "%s.%06ld%s",
            ^~
../lib/audit_logging/audit_logging.c:606:3:
   tz);
   ~~
../lib/audit_logging/audit_logging.c:600:2: note: ‘snprintf’ output
       between 8 and 70 bytes into a destination of size 50
  snprintf(
  ^~~~~~~~~
   timestamp,
   ~~~~~~~~~~
   sizeof(timestamp),
   ~~~~~~~~~~~~~~~~~~
   "%s.%06ld%s",
   ~~~~~~~~~~~~~
   buffer,
   ~~~~~~~
   tv.tv_usec,
   ~~~~~~~~~~~
   tz);
   ~~~

BUG: https://bugzilla.samba.org/show_bug.cgi?id=13437

Signed-off-by: Andreas Schneider <asn@samba.org>
Reviewed-by: Guenther Deschner <gd@samba.org>
Autobuild-User(master): Andreas Schneider <asn@cryptomilk.org>
Autobuild-Date(master): Thu May 17 20:17:35 CEST 2018 on sn-devel-144