Add stream instance id to the packet header

Add stream instance id to the packet header

This new field allows the viewer to distinguish between trace files
belonging to the same packet stream (in LTTng: the same CPU in the same
channel).

This is a locked-step with the corresponding commit in lttng-tools.

Signed-off-by: Julien Desfossez <jdesfossez@efficios.com>
Acked-by: Mathieu.Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>