Fix: pthread::mutex unlock must not throw

Fix: pthread::mutex unlock must not throw

unlock() is often called by destructors (e.g. lock guard); it must not
throw. We don't expect unlock to fail given our current usage anyhow.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I5dfd856f8a2dd29fd7c480c6ab6289b5b91c4391