fix typo.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@12558 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

lib/mkmf.rb

@@ -102,7 +102,7 @@ def install_dirs(target_prefix = nil)
       ['ARCHHDRDIR',    '$(rubyhdrdir)/$(arch)/ruby$(target_prefix)'],
     ]
   else
-n    dirs = [
+    dirs = [
       ['RUBYCOMMONDIR', '$(sitedir)$(target_prefix)'],
       ['RUBYLIBDIR',    '$(sitelibdir)$(target_prefix)'],
       ['RUBYARCHDIR',   '$(sitearchdir)$(target_prefix)'],