libcamera: pipeline_handler: Add accessor for useCount_

Add an accessor for useCount_ parameter, so that PipelineHandler
child classes can access it to verify whether the media device
is already locked or not.

Signed-off-by: Antoine Bouyer <antoine.bouyer@nxp.com>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Barnabás Pőcze <barnabas.pocze@ideasonboard.com>
[Kieran: fix extraneous ;]
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

src/libcamera/pipeline_handler.cpp

@@ -811,6 +811,12 @@ void PipelineHandler::disconnect()
  * \return The pipeline handler name
  */
 
+/**
+ * \fn PipelineHandler::useCount()
+ * \brief Retrieve the pipeline handler's used camera count
+ * \return The number of acquired cameras of the pipeline handler
+ */
+
 /**
  * \fn PipelineHandler::cameraManager() const
  * \brief Retrieve the CameraManager that this pipeline handler belongs to