std::sort needs <algorithm> to be included

ngx_pack.cc: In static member function ‘static void google::protobuf::compiler::nginx::Generator::GeneratePack(const google::protobuf::Descriptor*, google::protobuf::io::Printer&)’:
ngx_pack.cc:330:3: error: ‘sort’ is not a member of ‘std’
   std::sort(extensions.begin(), extensions.end(), ExtensionRangeSorter());
   ^