Add vendored/interface target for quatlib.
Add vendored/interface target for quatlib.

OSVR/OSVR-Core,godbyk/OSVR-Core,godbyk/OSVR-Core,OSVR/OSVR-Core,OSVR/OSVR-Core,OSVR/OSVR-Core,godbyk/OSVR-Core,godbyk/OSVR-Core,godbyk/OSVR-Core,OSVR/OSVR-Core,godbyk/OSVR-Core,OSVR/OSVR-Core