common.mk: fix Unicode table dependency

* common.mk (.unicode-tables.time): needs Unicode files always,
  and should update after downloading these files.
  [ruby-core:66026] [Bug #10461]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48205 b2dd03c8-39d4-4d8f-98ff-823fe69b080e