Fix: the clock plugin must be initiated before first use of the clock

When building lttng inside the kernel the clock plugin must be initated
before the rest of the lttng code. Moved the module_init to
rootfs_initcall. The functionality will not change when built as a
module.

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