extconf.rb: libffi PIC objects

* ext/fiddle/extconf.rb: make PIC objects if it will be linked as
  a shared object eventually.  [ruby-core:67128]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@49007 b2dd03c8-39d4-4d8f-98ff-823fe69b080e