cubeb-api: Fix linker path after libcubeb submodule update.
cubeb-api: Fix linker path after libcubeb submodule update.

https://github.com/kinetiknz/cubeb/commit/871456d0 causes the library to
be generated in a lib subdirectory rather than in the root of the build
output, so update build.rs to search in the new location.

djg/cubeb-rs