bench/bench_lru: make it build again, and check it in CI

bench/bench_lru: make it build again, and check it in CI

It's not too important, but I just stumbled upon this and it looked like
nobody has touched it in a long time. Since it's not completely broken
(the fix is trivial), I decided to fix it and add it to CI to ensure
that we can still build it. Maybe `lru` will be used some more at some
point again in the future...