Does not build cleanly with `-Wsign-compare`

[adetaylor]

Experimenting with building bits of centipede with Chromium, which uses -Wsign-compare

../../third_party/centipede/src/runner.cc:598:29: error: comparison of integers of different signs: 'int' and 'uint64_t' (aka 'unsigned long') [-Werror,-Wsign-compare]
        rand_r(&seed) % 100 < state.run_time_flags.crossover_level) {

[adetaylor]

(obviously this is trivial to work around, I just thought I should let you know)

[kcc]

thanks will fix. (unless someone wants to beat me to that).
Good idea to also add -Wsign-compare to the build flags.