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.