include: sframe: rename sframe_func_desc_entry to use an explicit v2

As SFrame format evolves, this will be useful to refer to v2 on-disk
layout directly as the format evolves.

Reviewed-by: Jens Remus <jremus@linux.ibm.com>

include/
        * sframe.h (struct sframe_func_desc_entry): Rename to tag v2.