Require the powershell mixin explicitly

Race condition with requires sometimes produces this error:

````
$ ./msfconsole -L
[-] WARNING! The following modules could not be loaded!
[-] 	/home/todb/git/rapid7/metasploit-framework/modules/exploits/windows/http/oracle_endeca_exec.rb:
NameError uninitialized constant Msf::Exploit::Powershell
````

Fixed by explicitly requiring the powerhsell mixin.

## Verification

- [ ] Start `msfconsole`
- [ ] `info exploit/windows/http/oracle_endeca_exec` should produce results.

Note, expressing this bug tends to depend on path load orders, so often will not be triggered (eg, when Metasploit Pro / Community Edition loads, requires tend to be pulled in ahead of time).