Mark the find_package(benchmark) with required

Add the REQUIRED flag to the find_package command to find the Google benchmark package. Without that flag, the find_package command will happily pass if the benchmarks are not found.

However, if benchmarks is not found, then the CMake configure will then fail because the benchmark::benchmark library is not defined. That is a confusing error to get. Since benchmark is not a typical thing installed on a system, this can be problematic.