win32/registry.rb: use WCHAR

* ext/dl/win32/lib/win32/registry.rb (Error, API): use WCHAR
  interfaces.  c.f. [Bug #8508]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41838 b2dd03c8-39d4-4d8f-98ff-823fe69b080e