Include missing require for powershell

## Verification

 - [ ] Before patch, on Linux:

````
todb@mazikeen:~/git/rapid7/metasploit-framework
$ ./msfconsole -L
[-] WARNING! The following modules could not be loaded!
[-] 	/home/todb/git/rapid7/metasploit-framework/modules/exploits/windows/local/powershell_cmd_upgrade.rb: NameError uninitialized constant Msf::Exploit::Powershell
````

 - [ ] After patch: no errors