* common.mk: remove timestamp in distclean-ext realclean-ext.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40563 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2013-05-03

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40564 b2dd03c8-39d4-4d8f-98ff-823fe69b080e