portable-ruby: disable fiddle

This will opportunistically pick up a Homebrew libffi (not available
in the OS on all platforms), and isn't used by anything that we're
using this portable Ruby for.

Formula/portable-ruby.rb

@@ -26,7 +26,7 @@ def install
       --enable-load-relative
       --with-static-linked-ext
       --disable-dln
-      --with-out-ext=tk,sdbm,gdbm,dbm,dl,coverage
+      --with-out-ext=tk,sdbm,gdbm,dbm,dl,coverage,fiddle
       --disable-install-doc
       --disable-install-rdoc
       --disable-dtrace