Added a C-API lock for Nokogiri.

This makes whatever the issue is in

https://github.com/sparklemotion/nokogiri/issues/1047
https://github.com/rubinius/rubinius/issues/2844

to not manifest during more than 1 hour of 100 threads running in
the repro script in the Nokogiri ticket.

If a bug never appears, does it exist?