virtual_oss is part of base from 15.0 onwards, as of src commit FILLME
("virtual_oss: Port to base"), so instead of depending on
audio/virtual_oss for those versions, build using the base system
sources instead.
While here, since audio/virtual_oss' upstream has been moved to
FreeBSD's GitHub, use that in GH_TUPLE. Also use the latest version.
Sponsored by: The FreeBSD Foundation