ripper/depend: fix out-of-place build

* ext/ripper/depend (ripper.o): fix out-of-place build.  probes.h
  depends on the platform, so it cannot be in $(top_srcdir).

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40272 b2dd03c8-39d4-4d8f-98ff-823fe69b080e