Add 'contrib/libsamplerate/' from commit '69a539a327ef8d78260056c5394363569b5751ef'

Initial import. Needed by upcoming patches which port virtual_oss to
base.

Sponsored by: The FreeBSD Foundation

git-subtree-dir: contrib/libsamplerate
git-subtree-mainline: e9dd9f95f82f6244a6eafd073531dcf6b6a3953d
git-subtree-split: 69a539a327ef8d78260056c5394363569b5751ef