Avoid require race with powershell

The Powershell exploit lib needs to be required directly, otherwise differences in load order will cause a missing constant error.

### Verification

- [ ] `./msfconsole -Lq -x "sleep 3; info exploit/windows/local/ms13_005_hwnd_broadcast"`
- [ ] See that the module is loaded.

Note, the error condition may or may not reproduce for you, it depends largely on ruby system path ordering.