extractor.cc: warning on linux build [JIRA: RIAK-2300]

[matthewvon]

Compiling c_src/extractor.cc
c_src/extractor.cc: In member function ‘virtual void ExtractorMsgpack::extract(const char_, size_t, ExpressionNode_)’:
c_src/extractor.cc:541:22: warning: comparison between signed and unsigned integer expressions [-Wsign-compare]