lib/virtual_oss: don't (over)link to libsamplerate

These plugins require samplerate.h due to virtual_oss's int.h including
it, but don't use any symbols directly so don't link to the library.

Centralize adding the include path.

Reviewed by: christos
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D54705