ruby.pc.in: rubyarchhdrdir

* template/ruby.pc.in (Cflags): use rubyarchhdrdir for multiarch.
  [Bug #7874]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@40580 b2dd03c8-39d4-4d8f-98ff-823fe69b080e