Add requires to portproxy

TheColonial spotted it:

```
18:11 < TheColonial> Is anyone seeing this on MSF/master:
18:11 < TheColonial> [-] WARNING! The following modules could not be loaded!
18:11 < TheColonial> [-]     /home/oj/code/metasploit-framework/modules/post/windows/manage/portproxy.rb: NameError uninitialized 
                     constant Msf::Post::Windows

```

Hope this pull request fixes the problem until #2354 is landed