streamedbuffer: use correct priv_size
streamedbuffer: use correct priv_size

StreamedBuffer nodes are based on struct buffer_priv not struct
variable_priv.

This was not causing any issue as the size of the variable_priv
structure (368 bytes) is larger than the size of the buffer_priv one
(200 bytes).

gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl,gopro/gopro-lib-node.gl