common.mk: fix exec
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51336 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
common.mk: fix exec
@nagachika (Tomoyuki Chikanaga).
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@51336 b2dd03c8-39d4-4d8f-98ff-823fe69b080e