audio/virtual_oss: Enable BT_SPEAKER by default

There is no good reason not to have this option disabled by default.
Most people nowadays want bluetooth support as well so it's good to have
it enabled.

PR: 203745
Requested by: Zahemszky Gábor <gabor@zahemszky.hu>
Sponsored by: The FreeBSD Foundation
MFC after: 2 days
Reviewed by: lwhsu, emaste
Differential Revision: https://reviews.freebsd.org/D46376