mixer(8): Update virtual_oss path

virtual_oss is part of base as of 9cab9fde5eda ("virtual_oss: Port to
base").

Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D52322

(cherry picked from commit 5c8835e5c0ce34ee50eba92bb5f4de36139ae7ea)