Silence bogus Coverity warning of uninitialized value

Coverity complains that struct lttng_trigger_comm's
payload field is not initialized. "payload" is a flexible
array member which should be ignored. Normally, a such a
member _could_ result in additional padding added at the
end of the structure. In this case, the structure is declared
as packed.

Nonetheless, using a designated initializer gets rid of the
ambiguity and silences Coverity.

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