Fix bad namecoin config lookup on OSX

When starting on OSX with the new namecoin integration, this error message flies by in the console:

    Failure reading namecoin config file: global name 'os' is not defined

After adding the require, I get the proper error indicating I've never configured namecoin settings:

    Failure reading namecoin config file: [Errno 2] No such file or directory: '/Users/grant/Library/Application Support/namecoin/namecoin.conf'