Nbns spoof  fix EADDRINUSE issue

Modified nuns_response.rb to fix the following error in OSX 10.8

msf auxiliary(nbns_response) > run
[*] Auxiliary module execution completed

[-] Auxiliary failed: Errno::EADDRINUSE Address already in use - bind(2)
[-] Call stack:
[-]   /Users/r3dy/dev/pentestgeek/metasploit-framework/modules/auxiliary/spoof/nbns/nbns_response.rb:62:in `bind'
[-]   /Users/r3dy/dev/pentestgeek/metasploit-framework/modules/auxiliary/spoof/nbns/nbns_response.rb:62:in `run'

Tested the modifications and it still works in Kali Linux.