Fix: event: format specifier for ssize_t

Fix: event: format specifier for ssize_t

Observed issue
==============

  In file included from event.cpp:15:
  event.cpp: In function ‘ssize_t lttng_event_create_from_payload(lttng_payload_view*, lttng_event**, lttng_event_exclusion**, char**, lttng_bytecode**)’:
  ../../src/common/error.h:191:28: warning: format ‘%lu’ expects argument of type ‘long unsigned int’, but argument 4 has type ‘ssize_t’ {aka ‘int’} [-Wformat=]
    191 |  __lttng_print(PRINT_WARN, "Warning: " fmt "\n", ## args)
        |                            ^~~~~~~~~~~~~~~~~~~~
  ../../src/common/error.h:139:51: note: in definition of macro ‘__lttng_print’
    139 |    fprintf((type) == PRINT_MSG ? stdout : stderr, fmt, ## args); \
        |                                                   ^~~
  event.cpp:624:4: note: in expansion of macro ‘WARN’
    624 |    WARN("Userspace probe location from the received buffer is not the advertised length: header length = %" PRIu32 ", payload length = %lu", event_comm->userspace_probe_location_len, ret);
        |    ^~~~

Solution
========

Albeit there is no "canonical" way of printing ssize_t, use '%zd' since
we already make use of it elsewhere.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Id41e6ccf07bd580813f169b65d281a4fa305fb48