linux.ui: add kernel.swtbot as a friend

linux.ui: add kernel.swtbot as a friend

lttng.Kernel has the LTTng trace support needed to test these views.
That is why their unit tests are in the other package. Adding friend
allows access to the internal IDs instead of hard-coding them.

Change-Id: Ia93453ece165d26cba54b05dbf389765b37471a2
Signed-off-by: Matthew Khouzam <matthew.khouzam@ericsson.com>
Reviewed-on: https://git.eclipse.org/r/80693
Reviewed-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Tested-by: Genevieve Bastien <gbastien+lttng@versatic.net>
Reviewed-by: Hudson CI