* common.mk (clean-ext): remove timestamps.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40340 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2013-04-18

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40341 b2dd03c8-39d4-4d8f-98ff-823fe69b080e