Makefile.sub: gem_prelude to load rubygems

* win32/Makefile.sub (DEFAULT_PRELUDES): always use gem_prelude
  regardless of --disable-rubygems.  follow up r43278.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43282 b2dd03c8-39d4-4d8f-98ff-823fe69b080e