The fuse_models package needs an optional dependency on `benchmark`

[svwilliams]

The fuse_models package uses the benchmark library as part of the test build. That library is not available for Xenial/Kinetic. The CMake build system conditionally compiles the benchmark test correctly, but I know of no way to specify a conditional/optional dependency in the package.xml file. As such, adding the benchmark dependency to the fuse_models package.xml breaks the Xenial/Kinetic buildfarm build. See PR #124.