output: JSON: increase time-stamp buffer size

output: JSON: increase time-stamp buffer size

The output buffer for date-times is of sufficient size provided that we
don't get oversized integer values for any of the fields, which is a
reasonable assumption.  However, the compiler complains about possible
truncation, e.g.:

  ulogd_output_JSON.c:314:65: warning: `%06u` directive output may be truncated writing between 6 and 10 bytes into a region of size between 0 and 18
  ulogd_output_JSON.c:313:25: note: `snprintf` output between 27 and 88 bytes into a destination of size 38

Fix the warnings by increasing the buffer size.

Signed-off-by: Jeremy Sowden <jeremy@azazel.net>
Signed-off-by: Pablo Neira Ayuso <pablo@netfilter.org>