common.mk: fix build error

* common.mk (ext/ripper/ripper.c): VPATH is always ../.. from
  ext/ripper.

* ext/ripper/depend (.y.c): VPATH is not needed for an
  intermediate file.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48499 b2dd03c8-39d4-4d8f-98ff-823fe69b080e