libcamera: camera: Add a const version of the pipe() function

Allow access to the pipeline handler on a const instance of
Camera::Private.

Signed-off-by: Stefan Klug <stefan.klug@ideasonboard.com>
Reviewed-by: Jacopo Mondi <jacopo.mondi@ideasonboard.com>
Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>

src/libcamera/camera.cpp

@@ -603,6 +603,11 @@ Camera::Private::~Private()
  * \return The pipeline handler that created this camera
  */
 
+/**
+ * \fn Camera::Private::pipe() const
+ * \copydoc Camera::Private::pipe()
+ */
+
 /**
  * \fn Camera::Private::validator()
  * \brief Retrieve the control validator related to this camera