tools: re-add batched-bench to server-focused build

benchmark.sh's batched test suite requires the llama-batched-bench
binary, which was shedded in da9a6da82's server-focused build.
Re-add add_subdirectory(batched-bench) and fix the stale comment
that listed tts, rpc, and batched-bench as shedded when they are
actually built.