Fix: Add kernel configuration for lttng clock plugin

Fix: Add kernel configuration for lttng clock plugin

Only one lttng clock plugin can be used when building the lttng-modules
in the kernel. To make it possible to use a custom clock plugin it must
be possible to unconfigure the test clock plugin

Signed-off-by: Anders Wallin <wallinux@gmail.com>
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>