Clean-up: sessiond-comm: out of bounds access warning

gcc 11.2 produces the two following warnings. In both case, setting an
array's dimension to zero is used to express a variable length array of
names that are LTTNG_SYMBOL_NAME_LEN bytes long. gcc doesn't know about
this and correctly points out that an access is taking place outside of
the array's bounds.

Omit the '0' dimension to work around this warning.

event.cpp: In function 'ssize_t lttng_event_create_from_payload(lttng_payload_view*, lttng_event**, lttng_event_exclusion**, char**, lttng_bytecode**)':
event.cpp:320:62: warning: array subscript i is outside array bounds of 'char [0][256]' [-Warray-bounds]
  320 |                 ret = lttng_strncpy(local_exclusions->names[i],
      |                                     ~~~~~~~~~~~~~~~~~~~~~~~~~^
In file included from event.cpp:16:
../../src/common/sessiond-comm/sessiond-comm.h:569:14: note: while referencing 'lttng_event_exclusion::names'
  569 |         char names[0][LTTNG_SYMBOL_NAME_LEN];
      |              ^~~~~

event-rule/user-tracepoint.cpp: In function 'lttng_event_rule_generate_exclusions_status lttng_event_rule_user_tracepoint_generate_exclusions(const lttng_event_rule*, lttng_event_exclusion**)':
event-rule/user-tracepoint.cpp:383:61: warning: array subscript i is outside array bounds of 'char [0][256]' [-Warray-bounds]
  383 |                 copy_ret = lttng_strncpy(exclusions->names[i], exclusion_str,
      |                                          ~~~~~~~~~~~~~~~~~~~^
In file included from ../../src/common/runas.h:17,
                 from event-rule/user-tracepoint.cpp:17:
../../src/common/sessiond-comm/sessiond-comm.h:569:14: note: while referencing 'lttng_event_exclusion::names'
  569 |         char names[0][LTTNG_SYMBOL_NAME_LEN];
      |              ^~~~~

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I260185f2baf085ca4486ce3b13696ee5fa55938a