libcamera: buffer: Plane: Drop friend statement

The Plane class do not need to friend Stream, drop it.

Signed-off-by: Niklas Söderlund <niklas.soderlund@ragnatech.se>
Reviewed-by: Jacopo Mondi <jacopo@jmondi.org>
Reviewed-by: Kieran Bingham <kieran.bingham@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>

include/libcamera/buffer.h

@@ -29,8 +29,6 @@ public:
 	unsigned int length() const { return length_; }
 
 private:
-	friend class Stream;
-
 	int mmap();
 	int munmap();