audio/virtual_oss: Start service after ldconfig

If virtual_oss is built with sndio enabled, the service might not start
successfuly on boot if ldconfig has not already updated the shared
library cache.

Proposed by: atagster@gmail.com
PR: 287198
Sponsored by: The FreeBSD Foundation
Reviewed by: arrowd
Differential Revision: https://reviews.freebsd.org/D50635