WorkQueue: avoid potential semaphore counter overflow

This could happen in the following cases:
- IRQ/publisher rate is faster than the processing rate, and therefore
  WorkQueue::Add is called at a higher rate
- a long-running or stuck task that blocks the work queue a long time

Both cases are not expected to happen under 'normal' circumstances (if the
system runs as expected).

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

@@ -76,6 +76,8 @@ private:
 
 	bool should_exit() const { return _should_exit.load(); }
 
+	inline void signal_worker_thread();
+
 #ifdef __PX4_NUTTX
 	// In NuttX work can be enqueued from an ISR
 	void work_lock() { _flags = enter_critical_section(); }