os.linux: Make KernelCtfTraceStub public

os.linux: Make KernelCtfTraceStub public

Other tests might want to make use of kernel traces without
depending on the lttng2.kernel plugins.

One limitation is that that stub uses the DefaultEventLayout,
which means only very early LTTng 2.x traces will work. More
recent event layouts are only defined in the lttng plugins
at the moment.

Change-Id: I0717cba18e696cabb4bbad53c90dbe471f836ffa
Signed-off-by: Alexandre Montplaisir <alexmonthy@efficios.com>