Add token to `struct lttng_kernel_event`

Add token to `struct lttng_kernel_event`

This token is provided by the user when registering an event rule to the
kernel. It is going to used to identify messages from event notifiers, and
counter bucket owners in the upcoming event notifier and map counter
features.

Signed-off-by: Francis Deslauriers <francis.deslauriers@efficios.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Change-Id: I58cd6b6e33b97cc21e50cc0b36bef4b9e4224423