mkmf.rb: revert DESTDIR prefix

* lib/mkmf.rb (install_dirs): revert DESTDIR prefix by r39841, since
  it is fixed by r41648.  [ruby-core:55760] [Bug #8115]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@41756 b2dd03c8-39d4-4d8f-98ff-823fe69b080e