common.mk: ignore clean-runnable errors

* common.mk (clean-runnable): get rid of errors running again after
  clean-runnable.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46707 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-07-06

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46708 b2dd03c8-39d4-4d8f-98ff-823fe69b080e