libcamera: buffer: Break friendship with Request

The FrameBuffer class is only friends with Request so that the request
can be associated with the buffers.

FrameBuffer already has a helper to setRequest(), so let's use that
directly instead.

Reviewed-by: Paul Elder <paul.elder@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Kieran Bingham <kieran.bingham@ideasonboard.com>

include/libcamera/buffer.h

@@ -56,7 +56,6 @@ public:
 private:
 	LIBCAMERA_DISABLE_COPY_AND_MOVE(FrameBuffer)
 
-	friend class Request; /* Needed to update request_. */
 	friend class V4L2VideoDevice; /* Needed to update metadata_. */
 
 	std::vector<Plane> planes_;