ScheduledWorkItem::ScheduleClear: remove item from runnable queue

The existing behavior is unexpected: if the work item is already on the
runnable queue, it will still be triggered after a call to ScheduleClear().
This can lead to race conditions.

platforms/common/include/px4_platform_common/px4_work_queue/WorkItem.hpp

@@ -81,6 +81,10 @@ protected:
 
 	virtual ~WorkItem();
 
+	/**
+	 * Remove work item from the runnable queue, if it's there
+	 */
+	void ScheduleClear();
 protected:
 
 	void RunPreamble() { _run_count++; }