Add missing includes to step-17 and step-18

This commit fixes up a regression introduced with commit
  06a70bad3cbbcbb6abc51e12f29e3b5dfffb928a
that removed unnecessary includes of base/multithread_info.h. Both steps
need them for multithread_info.n_threads()
Merge pull request #26 from tamiko/fix_includes_in_steps

Matthias: This commit fixes up a regression introduced with commit
06a70ba
that removed unnecessary includes of base/multithread_info.h. Both steps
need them for multithread_info.n_threads()