fiddle/extconf.rb: supply 0

* ext/fiddle/extconf.rb: supply 0 to fill RUBY_LIBFFI_MODVERSION
  with 3-digit.  libffi 3.1 returns just 2-digit.
  [ruby-core:62920] [Bug #9897]

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46485 b2dd03c8-39d4-4d8f-98ff-823fe69b080e