common: silence bogus coverity warning

common: silence bogus coverity warning

Coverity reports:
  1512891 Uninitialized scalar variable
  The variable will contain an arbitrary value left from earlier computations.

  In lttng_event_serialize(lttng_event const *, unsigned int, char
  const * const *,   char const *, unsigned long, lttng_bytecode *, lttng_payload *): Use of an uninitialized variable (CWE-457)

This warning is bogus since lttng_event_exclusion_comm contains a single
field which is already initialized and is packed (no padding possible).

Initialize the header explicitly to silence the warning.

Reported-by: Coverity Scan
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: Ia1eeee779168b3ac0eb9d0796d503b2d9ab225f2