Silence bogus Coverity warning of uninitialized value

Coverity complains that struct lttng_evaluation_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>