Support SRVHOST option for exploits/windows/browser/msvidctl_mpeg2.rb

For https://github.com/rapid7/metasploit-framework/issues/4133

To verify this, this is what I did:

- [x] Put a ```print_debug(host.inspect)``` at line 198
- [x] Run the module
- [x] In another terminal, ```curl -v``` the exploit's uri
- [x] ```curl -v``` will tell you where you should redirect to. Just follow that and curl again.
- [x] The ```print_debug``` should trigger and print either SRVHOST or Rex::Socket.source_address depending on how you set it. If you change SRVHOST to something else other than 0.0.0.0, then the module will use that. Otherwise it will fall back to Rex::Socket.source_address.
- [x] When you're done testing, make sure you undo the code change before landing.