windows: don't compute nfds for select() on Windows
windows: don't compute nfds for select() on Windows

Because `nfds` is ignored on Windows.

cosmo0920/groonga,hiroyuki-sato/groonga,cosmo0920/groonga,groonga/groonga,kenhys/groonga,kenhys/groonga,naoa/groonga,kenhys/groonga,kenhys/groonga,komainu8/groonga,groonga/groonga,cosmo0920/groonga,redfigure/groonga,komainu8/groonga,groonga/groonga,hiroyuki-sato/groonga,cosmo0920/groonga,hiroyuki-sato/groonga,naoa/groonga,redfigure/groonga,naoa/groonga,hiroyuki-sato/groonga,naoa/groonga,hiroyuki-sato/groonga,hiroyuki-sato/groonga,cosmo0920/groonga,naoa/groonga,kenhys/groonga,komainu8/groonga,cosmo0920/groonga,naoa/groonga,cosmo0920/groonga,komainu8/groonga,cosmo0920/groonga,redfigure/groonga,kenhys/groonga,redfigure/groonga,komainu8/groonga,redfigure/groonga,groonga/groonga,redfigure/groonga,redfigure/groonga,groonga/groonga,kenhys/groonga,redfigure/groonga,naoa/groonga,groonga/groonga,komainu8/groonga,naoa/groonga,groonga/groonga,komainu8/groonga,hiroyuki-sato/groonga,kenhys/groonga,hiroyuki-sato/groonga,groonga/groonga,komainu8/groonga