Fix: getcpu/clock plugin handle leak

Fix: getcpu/clock plugin handle leak

Keep the handle to the shared object in a static variable. Use it to
check whether it has already been initialized.

We don't allow dlclose() for now though, so it technically leaks, but at
least we keep a reference to the handle.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>