libsamplerate: Only build when virtual_oss is built

Gate libsamplerate behind MK_CUSE and MK_SOUND, like virtual_oss.
Nothing else uses this library, so there's no point building it
if we aren't building virtual_oss.

This avoids building a useless FreeBSD-sound package containing
only this library when WITHOUT_SOUND is set.

MFC after: 1 week
Reviewed by: christos, emaste
Differential Revision: https://reviews.freebsd.org/D56164
Sponsored by: https://www.patreon.com/bsdivy