common.mk: update unicode data only if BASERUBY is available

* common.mk (lib/unicode_normalize/tables.rb): do nothing unless
  BASERUBY is available.  MINIRUBY cannot load extension libraries,
  so cannot update Unicode data.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48278 b2dd03c8-39d4-4d8f-98ff-823fe69b080e